Access OpenVPN server behind T-mobile Home Internet

My first question is, which gateway are you using? Is it the Arcadyan KVD21 5G gateway?

There is currently no way to create a NAT back to your environment with T-Mobile. That's because the type of Internet you're getting from them is Fixed Wireless Access (FWA), which fundamentally differs from a traditional ISP like Xfinity. With a conventional ISP, you get a geographically local external IP address. With FWA, you don't. For example, I live in S. Florida, but my external IP is in Seattle, WA. I get around this by using a VPN on the network device where I need it and connecting to a local server to get a local external IP.

Another problem with the KVD21 is that DHCP can't be disabled. That forces you to use your Asus in Access Point Mode (AP Mode) so you don't have two DHCP servers on the network.

If you must be able to create a NAT back to your local network, you might need something other than T-Mobile FWA. I'd also suggest you watch the Nater Tater videos on YouTube, which go into some of these problems and how they might be addressed.

I hope this helped. Good luck!