5G Home Internet went from good to weak

I recently had to move my gateway to a different location in my house because of some change made by T-Mobile. For some reason (they said that some "modernization" is being done on the tower) the signal strength dropped from good to weak resulting in slower WiFi speeds. Moving the gateway got the good signal back but WiFi was still slow to my Roku TV because of the distance from the gateway. So I installed a TP-Link Archer A6 router and a TP-Link RE615X range extender. Speed to my TV is better but not as good as in the room with the gateway. Also, upload speed from the router is considerably slower than from the gateway. I've tried the router in both router mode and in AP mode. I've tried with the WiFi radios on the gateway turned on and turned off. I must setting this up incorrectly as I don't see why the WiFi from the gateway is faster than from the router. Any suggestions will be appreciated.