NetgearEX3700

Are you attempting to extend both the 2.4 GHz and 5GHz or only the 5GHz? You can try the manual setup to extend only the 2.4 GHz vs. 5GHz and see if that works vs having it try to do both bands. Since it is an AC vs AX it may have something to do with how the older extender handles the two bands. If it is the vintage with 802.11ac wave1 then it will only have the channel width 20, 40, 80MHz where the 802.11ac wave2 handles the channel width 20, 40, 80, 160MHz as the 802.11ax does. 

So, it is hard to say without knowing the exact vintage of the extender. It also can function in AP mode if it is connected with an Ethernet cable. 

I don’t have one of these so I am just looking at what might be a factor.

ScottyMac wrote:

Just switched to T-Mobile Home Internet and have been trying to connect a Netgear EX3700 to the router.  It's a no go.  I spoke with T-Mobile support who said that it's compatible but I've tried 20 times and it just won't hook up.   Can't get support from Netgear because I purchased this over three years ago.  Any help would be greatly appreciated.

It may not support both 2.4 and 5 ghz bands sharing the same SSID. Try to change your Tmobile gateway wifi into 2 separate SSID names and see if that works. Use the web UI to make this change on the Tmobile gateway.

Also, make sure you are using the manual setup on your Netgear and not WPS since Tmobile gateway doesn’t support WPS.