How to configure WiFi Settings like Channel, bandwidth in TM Gateway Model: TMO-G4AR?

There is a third-party app called HINT Control (available for iOS and Android) that lets you select a specific channel (or Auto) for both 2.4 GHz and 5 GHz.  It also lets you select a specific bandwidth (such as 20 MHz or 40 MHz, or Auto) for both 2.4 GHz and 5 GHz.  

Regarding the cellular bands, I don't think there is a way to force it to select a band. Mine (Sagemcom FAST 5688W) always uses n41. HINT Control shows n25, n41, n48, n66, n71, and n77 as Supported. So I assume the gateway picks "the best" one.

I have had mine for over a year, and almost all my devices have worked perfectly on the default network as configured out of the box. I only needed a second 2.4 GHz network with reduced security for my ancient desktop computer and printer.  My download speed is typically over 400 Mbps.