Router Configuring Advice

Good to hear you have most of what you were looking for working. To answer some of your questions:

AP mode - this should be seamless to enable. I've switched back and forth between DHCP and AP mode and never lost wifi connection after the router took a few minutes to auto reboot. When switching to AP mode, it takes another min or 2 for the TMO router to assign all the new IPs. However, if you set any static IPs back in DHCP mode, those will not work since the static IPs were on a different subnet.

DHCP mode - this allows your Asus router to do everything it's suppose to do. If you don't have any connectivity issues with your apps, then I would recommend you keep it this way. I haven't had any problems either and as you said no change in bandwidth or ping.

Web UI access to Asus Router - in AP mode, you have to see what IP address the TMO router assigns to your Asus Router then use that IP address to access the web UI. In DHCP mode, you can use the standard "router.asus.com" URL since that will auto resolve in DHCP mode.

DNS - default for Asus in DHCP mode is "ISP Provided" which is the same as TMO DNS. You can try the various free DNS services to see if you get any better results. I know for sure if I don't change DNS from default, I am unable to reach my closest TMO server on Speedtest. There could be other websites that fail, but I don't use/visit.

MTU - you can try manually setting that on your PC/Laptop end. I did not notice any difference when I tried because whatever the difference was within the margin of variability of back to back Speedtest.

Merlin Firmware - I have not tried since it’s not available for my mesh system.