Mesh question

You can technically use any other 3rd party router. You have to decide which mode you want to use in this configuration:

1. AP mode - your 3rd party router will only setup wireless channels to use but that's about it. All other networking features will be managed by the Tmobile router. For example - static IPs, DNS, QoS that most routers allow you to manage will not work in this mode because the Tmobile router does not have these features. At least all of your devices will be on the same subnet. This is basically what the $10/month option gives you rom Tmobile.
2. DHCP mode - this creates a new subnet behind your 3rd party router that allows you to use whatever advanced features come with the 3rd party router. The downside is you have 2 NAT layers because Tmobile router does not support bridge mode. While most devices and online services will work fine, folks have reported gaming devices and services having issues. Worth a try since you can always go to AP mode if you experience problems.

One convenience of the Tmobile mesh is that it uses the same wifi settings as a router. If you go with a 3rd party mesh, it's best to put all of your devices on the 3rd party router's wifi and not use the Tmobile gateway wifi since you can't share the wifi SSIDs across the two devices anyway.

I have the Asus Zen mesh setup in DHCP and I have no problems with my internet needs, but I don't play online games. I have Youtube TV, Apple TV, Hulu (not live TV), Netflix, Disney+ and all of them work fine. I also use Nord VPN and a corporate VPN from a Fortune 100 company and those work fine as well.