Home Internet - I do not recognize a device on my network

You made me look closer: If you've had any device connected even once, it's added to that list.  I have the MAC of my PC that I had connected only once via ethernet in the list.

It’s apparently a historical list, not a current list.

Try  resetting the device.

I assume you've gotten the MAC IDs off each of your devices and compared? Maybe there's a device you hadn't considered like a smartwatch or somesuch?

MAC IDs are inherent to the device and cannot be changed. They can be spoofed, but I highly doubt T-Mobile would have a need to do that.

The router could assign a new IP when you connect, I haven’t paid much mind to that as it usually doesn’t matter.

The MAC address for the mystery device does not match the MAC for any of my devices.

I looked up the MAC for the mystery device online, and learned that this oneis a randomized number, not the useful/true MAC of a device.

I also read that MAC randomization is the default setting for iOS 14 (which I am using). So, maybe my own iOS 14 device generated this randomized MAC and the TMobile HI app is keeping it listed as an offline device (instead of purging it from the device list).

If the TMobile HI app continues to display all MACs that have ever connected to my network, then I guess the list will grow and grow if my iOS 14 device generates a new random MAC every time it connects. Time will tell.

Does this make sense?

Was about to post another reply about this.  Yes that's very likely what it is.

Randomized MACs are intended to make it harder to track your device as it connects to various WiFi networks.

I'm a software guy by trade, and my concern is exactly that purging. There doesn't seem to be a way of doing manually.  One hopes that there's some timestamping and autodeletion of those dead MAC IDs.

Can probably do an experiment of reducing the device limit to something below your current list length and see what the gateway does with it.

If iOS 14 generates a new randomized MAC if the SSID name is changed, then I believe that my mystery is solved.

I did change the name of my SSID after initial setup of the gateway, so maybe that name change caused my iOS 14 device to generate a new MAC? I could test it by changing my SSID back to the previous name and seeing what happens.

From what I read online, my iOS 14 device will use the same randomized MAC when reconnecting to a previously connected SSID. So, I do not expect the app to grow and grow the list of MACs. It would be nice to be able to purge the list after an SSID name change if that really does lead to new MACs (and rendering previous random MACs obsolete).

I cannot find the answer online - Is a new randomized MAC generated for an SSID name change? Even though the physical gateway is unchanged, does iOS 14 consider an SSID name change to be enough to cause it to generate a new MAC?

Thanks in advance for your knowledge and help.

I wonder if the High Speed Gateway I received yesterday was previously used, returned, and given to me.

There are two phones listed in Devices;  a Sony Experia C3 and another named Max's iPhone.   Neither are mine.   I changed the Wifi network name, and admin and wifi passwords.  Restarted the Gateway.   

Both phones were still listed.   I was able to disable the Experia, but not the iPhone that returned an error.

TMo not even factory resetting returned devices?! If this is the case, this is beyond improper conduct on their part, seriously so. 

OK, so I figured out the the phone reported by the Gateway App as Max's iPhone is actually my iPhone that is named Pro Max (not very inventive on my part).  Mac address matches. So maybe some gateway code thinks Max is my name?  LOL

But the Experia phone Mac address does not match any of my devices.  I was able to mark it disabled and so far has stayed that way.

I may try a reset, but the gateway is working so well that I don't want to tempt fate.   I am getting 500 Mbps down and 80 Mbps up with Speedtest.net App from my iPhone and 300 Mbps down testing from Chrome browser on my laptop.  I was going to keep my Mediacom Cable internet active for an extra month but there no longer seems to be a reason.

I encountered the MAC randomization issue with my iPhone 12 pro and my iPad Pro both where I could connect to the T-Mobile Nokia router and the wireless would show full connectivity on the WLAN but no internet connectivity. This appeared to be random but upon deeper investigation I discovered articles that explain in more detail how the MAC randomization works. Apparently iOS 14 generates a new random MAC address every 24 hours. (per macaddress.io FAQs) It is a very frustrating issue. My solution, which resolved the behavior as far as I can tell now, was to disable the feature and NOT use it on my local wireless LAN. The improvement in behavior was pretty much immediate after rejoining the wireless network. I also noticed in the Nokia router it has 128 devices per radio. If there is a table that builds persistent then I was maybe hitting the ceiling and that would account for why the problem became worse over time? I cannot say for sure about that but it is clear to me when I disabled the feature in the iOS 14 device my wireless network sessions became rock solid. I also discovered Android, Linux and Microsoft each also support MAC randomization. For Apple iOS 14 that security feature to prevent tracking is enabled by default. For anyone connecting an iOS 14 device onto a LAN where the MAC authorization is used this would have repercussions especially if the MAC is randomized every 24 hours as the FAQs suggest. “However, since a specification on MAC address randomization does not yet exist, iOS, Windows, and Linux, all implemented their own variants of MAC address randomization.” (per macaddress.io) I hope this helps to explain some of the frustrating behavior with iOS 14 devices.