Cant connect to Synology RT2600ac

I think the tech support guy you spoke with isn't very good.  I say this because "802.11ax" and "802.11ac" are hardware specifications that can't be changed with a setting.  Both support both 2.4 GHz and 5 GHz bands.  

If you were using this router with your previous ISP, and if you can connect the router to the gateway with an Ethernet cable, that should be all you need to do.  

If you are trying to connect the router to the gateway via Wi-Fi, I think you need to configure the router as a Range Extender (but I am not certain.)  It seems to me that if you connect via Wi-Fi to the default T-Mobile home internet network, which is both 2.4 GHz and 5 GHz networks, your router should broadcast both, but I don't know.

I hope another helper will jump in here.  Otherwise, you might need to call Synology again and hope for a different support person.  

EDITED TO ADD:  It took me a long time to compose my reply, and then I saw @copz1998 had already replied.  I’ll leave mine here though, hoping it might be of some value.