TLDR: I added a 2nd VPN server to my private network and when using that server there is no problem. In my view this tends to shift the blame away from the Microsoft Remote Desktop APP and towards the VPN server itself (but that is debatable).
I wanted to circle back and provide new information regarding the problem I have experienced using Microsoft’s Remote Desktop iPhone App when accessing my network via a VPN over a T-Mobile wireless connection in light of some new testing results.
Specifically: when using Microsoft Remote Desktop App version 10.4.5 (5227) on my iPhone and a T-Mobile wireless connection with a VPN connection into my home network:
Microsoft Remote Desktop fails returning Error code: 0x204 using ‘VPN Server A’ but connects properly with ‘VPN Server B’ (see below).
Interestingly, if I instead use my Pixel 4a (5G)’s Microsoft Remote Desktop version 8.1.81.435 and a T-Mobile wireless connection and make a VPN connection into my home network:
Microsoft Remote Desktop connects properly using either VPN server.
VPN Server A: ASUS RT-AC68U Router’s (Firmware 3.0.0.4.386_49703) built-in OpenVPN Server
VPN Server B: OpenVPN server version 2.5.5 x86_64-pc-linux-gnu running on a Windows 10 Hyper-V hosted Ubuntu 22.04 LTS Virtual Machine.
This second VPN server works perfectly well although I'm required to this Windows 10 computer (and the virtual Ubuntu machine) powered on to insure availability of my private network via Remote Desktop. At some point I'll check with the folks at ASUS and see if they are willing to work on this issue (assuming they can reproduce it).