How to use High-Speed Internet Gateway with Existing Router

I have a TP-Link Archer A7 router, and was able to set it to Access Point (AP) mode via the router Admin settings function. 
Not sure if I really needed to do that, but my router has the following statement about the AP mode; "In this mode, the router connects to a wired or wireless router via an ethernet cable and extends the coverage of your existing network. Functions like NAT, parental controls, and QOS are not supported in this mode"

I believe the benefit of having a router connected in addition to the T-Mobile home internet router is that they can be placed in different areas of the home, and then you can choose which device to connect to based on your signal strength.

I can still log into my router, I just need to be sure I’m logged into the router connection when I do it, rather than the T-Mobile connection.

I just got the T-Mobile Arcadyan rectangular KVD21 Gateway and it seems to be comparable to my Xfinity cable lowest tier 30 GB plan, although the speeds vary at times to much faster and much slower. This is only day 2 so I have time to decide on whether to keep it.