Cant connect to Synology RT2600ac

Thanks copz1998 and nc1037:  There some more information I need to provide. I am switching over from Startlink due to cost. The Synology router has been configured to run in Repeater mode while I have been on Starlink for 2 years and is configured like that as I switch to the Tmobile 5G GW. It has worked fine on StarLink which is Wifi5 I believe. En Repeater mode, the DHCP server along with a lot of features are disabled, i.e. the parent GW hands out IP addresses. The Wifi diversity and beamforming functionality still operate by default. Since the Tmobile 5G GW is on the second floor and my office space is on the first, there is no wired ethernet and I don't plan on any, so wireless is the only way to go. 

I forgot to mention that I split the Wifi bands on the TMobile 5G GW each with a unique SSID, so I could control the reconfig on the RT2600ac. So after a "reset to factory settings", I am able to configure the TMobile 5G GW 2.4Ghz band successfully, i.e. the RT2600ac searches for potential parents and displays a list from which I can see and select the TMobile GW 2.4GHz SSID. If I stop at this point and do not attempt to config the 5Ghz parent, every thing is copacetic.  If however, I search for 5Ghz parents, the unique SSID I configured he TMobile 5G GW is not found. I even added the SSID as if it was hidden (an option of the RT2600ac) and it says "searching" and the whole RT2600ac becomes unresponsive, requiring a hard reset to factory defaults to start over again. I have been through this process about 8 or 9 times.