Copatiible Wifi Extender with the TMobile 5G Internet Gateway?

If these are still working on your old network then that means the devices are likely using the old IP addresses and may even be static. Your old network is likely not on the same subnet as TMO gateway so you can try to factory reset your extenders and setup as new with TMO gateway. Another complication could be that TMO gateway combines by 5 and 2.4 ghz bands into one SSID and you will need to confirm if your extenders support that. If not, you need to manually separate the bands into 2 separate SSIDs using the web UI or TMI app. Also note that TMO gateway does not support WPS so you will need to setup the extenders with whatever app or web UI interface they have.