Unable to connect Chamberlain garage door opener via 5g internet

Mine is old NetGear N300 Model EX2700. I got it at least 5 year ago.

I do not think you need to buy a new extender. Your nighthawk extender should work. Your extender always tries to connect to the "Known" network it has connected to. You need to reset it to factory default setting. At that time, you will be able to pick Gateway network. You need to download a manual to follow the initial setup procedure after resetting to factory default. Trust me, t-mobile support would not know your extender better than you do. Incompatibility is an easy excuse to get you off the phone.

Your netgear should work also. But your extender might only work for 2.4 GHZ network as mine does. So you will need to separate 2.4 GHZ network out from default t-mobile gateway's hybrated network to create two networks for your t-mobile gateway. One for 2.4 GHZ, one for 5GHZ. At my home, 5GHZ is always much faster than 2.4 GHZ network. I have speed around 300 MBPS. So all my laptop/phone use 5GHZ network. But I leave this 2.4 GHZ for all the devices such as thermostat, smart devices. So far all works fine now.