Cant connect to Synology RT2600ac

No, my problem is ostensibly with the RT2600ac Router. It cannot see the 5Ghz Wifi band SSID of the 5G TMobile GW in its (the RT2600ac's) list of prospective parent GWs. The RT2600ac can see and connect to the 2.4 Ghz Wifi band of the 5G TMobile GW as a parent router. The 5G TMobile GW works great; my phone, 3 firesticks, and my wife's laptop can find and connect to either Wifi band with good throughput. The tech from synology recommended I down grade the 802.11 Mode from "ax" (Wifii6) to "ac" (Wifi5) to see if that fixes the problem. I need  the RT2600ac Router to provide wired ethernet to my Desktop. At least one other TMobile product, a 5G hotspot has the capability to change the 802.11 Mode from "ax". I was looking for a backdoor config capability of the 5G TMobile GW to achieve this type of change. I have concluded at this point that it is not possible and can live with the 2.4 Ghz Wifi  speed limitations.