Cant connect to Synology RT2600ac

I am unable to connect the 5Ghz WIFI band on my RT2600ac WiFi Router which is Wifi5 compatible. I contacted tech support at Synology re. this issue. he recommended I change the 802.11 mode from "ax" to "ac" for this to work. There does not appear to be a way to change this setting from the T Mobile Internet App. Form the internet, it appears other Wifi 6 compatible TMobile Gateways/Hotspots have the capability to change the "802.11 Mode". Is this possible with the TMO G4AR gateway?