Cannot access my Gateway at 192.168.12.1

Do you have a router connected to your gateway? What are those 192.168.68.xx IP addresses? If you're connected to a router which is giving out 192.168.68.xx IP addresses you will not be able to reach the gateway at 192.168.12.1.

Now that is curious. You have the firewall up or down? Odd that it was not working with the IP address but does not renew the IP address from the DHCP server on the router. I looked up the wireless driver and found a hit on it right away. The information is via the link below.

https://www.drivereasy.com/knowledge/fixed-qualcomm-atheros-qca61x4a-driver-issues-in-windows-10/

One other thing you might try is using an ethernet cable and plug the client direct to the router on one of the ethernet ports. If it can communicate out the Realtek interface this would be good to know. If not again I would disable the firewall and give it another try on the Ethernet port. This still might be something to do with the WIFI NIC driver. That it is an 802.11ac NIC it should be able to leverage both the 2.4GHz and 5GHz. Has it worked with the Nokia router since you started using the T-Mobile solution?

gpmaz - Good catch on the 192.168.68.105 address. It does appear that the client had an IPv4 address from another source. Now that is odd that it has the IP on the wrong subnet but sees the 192.168.12.1 DNS server. No way it will work picking up the IP address on the wrong subnet. That other DHCP server has to be disabled or it can mess up other clients.

iTinkeralot, It looks to me that he has a second DNS server pointed to 192.168.68.1. It that's the case he's likely not actually seeing the 102.168.12.1 DNS server. Far from an expert, but that's what it looks like to me.

My guess is that he plugged a router into the T-Mobile router, as you speculated, and it served the 192.168.12.1 DNS server up on its ethernet segment as the DNS server. No way clients on the second broadcast domain can ever be routed out and back as the T-Mobile router will never know of the ethernet segment on the other side of the other router. In effect he needs to turn that to bridge mode and disable the DHCP server or just shut the old router down. Unless there is a demand for expansion of the wireless to try to cover more area the best solution is to just go with the T-Mobile router. The second router would have the double NAT solution and well that could cause issues as well. 

From the IP config data we can see:

DNS Servers . . . . . . . . . . . : 192.168.12.1 & 192.168.68.1 so the 192.168.68.1 router must have picked up on the other DNS server from the T-Mobile router. Regardless clients on the 192.168.68.xx/24 segment are hosed. They can only talk to others on that segment so this makes sense now. The outside address WAN interface is probably on the ethernet of the T-Mobile router so it is on 192.168.12.x/24 on its WAN interface I would bet. 

I have a router plugged into my gateway. My 2 main computers, my printer and NAS are connected to the router. All other devices, cameras, streaming devices, phones, tablets, TVs etc are connected to the gateway. In my mind that separates the computers and NAS (which I don't stream from) from all the devices I don't really trust. I have a third computer which I rarely turn on which connects to the gateway should I ever need to access anything else including the gateway GUI. Seems to work fine for me.

So, do you run the same WIFI SSID and wireless security on both with WIFI radios active on both devices? I don't think we have the full picture of what has been done in his situation. Not yet at any rate. 

In your case your clients on the T-Mobile segment talk to one another and to the internet but unless the backend segment is advertised to the other side the clients would not be aware of it. A client from the internet could never reach into that back end segment as the T-Mobile router has no advertisement of the backend segment from your router. You would need static routing or a dynamic routing protocol on both routers so they could share routing information. I could see clients going out but nothing from the outside could initiate a conversation with one of those clients as it could not be found from the internet.

No, different SSID on the router. Devices on the gateway can't see devices connected to the router and vice versa. 

I could accomplish the same thing by using a guest network on the router but I get marginally faster speeds connected to the gateway and since I really don't want to go to the trouble of completely shutting off the Wifi on the gateway this approach seems simpler. Also not a gamer and no port forwarding, so the double NAT thing doesn't seem to pose a problem. That is of course until someone like yourself who very likely knows more about this than I do comes along and points out to me that I'm wrong. Sure wouldn't be the first time.

To the OP, sorry for hijacking your thread. Hopefully your PC is attached to a router and all you need to do is switch to the gateway's 191.168.12.xx network and you should have access. If you don't have a router attached to the gateway I'm at a loss as to where that other network is coming from.