RDC ISSUES

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).

Thank you Ale_rod1500,

Unfortunately using hostnames did not work for my VPN.  

As I mentioned before, our issue using the RD Client on Ipad accessing servers or computers via VPN was fixed as soon as we started using Hostnames instead of IP Addresses.

I.E:  PCname.domain.com.

I am having a similar issue.  I have tested iPad connected to my verizon hotspot, iphone as a hotspot, and two other backup cellular connections.  If I connect to verizon, or either of the two backups (all 3 of these have IP addresses that start with 192.168.1.....) and then connect to my company's VPN the RDClient app gives me the Error code: 0x204.  However, when I connect my iPad to my iPhone hotspot (ip 172.20.1......) and the VPN, it connects.  I mention the IPs because the desktops i'm attempting to connect to are also on a subnet that starts with 192.168.1.....  My Network Admin thought that may be playing a part of the issue.  In addition I tested these same 4 wifi connections with my windows laptop using the native RDP and all combinations worked.  
Its the iPad with RDClient & wifi in 192.168.1 subnet that cause the error.  I tried my host name instead of ip, but that did not work.
Thank you

I have the same issue but am able to connect with RDP using the Jump Desktop iOS app, so it appears to be an issue with the Microsoft Remote Desktop Mobile app.

not exactly, I have to go back to where the problem started: on  my new Ipad the RDC app didn't work when connected VPN using T-Mobile network to access my PC , after some research I noticed that the fix was to add my PC to the RDC App using the PC host name (instead of IP), the pc name found in Windows - Control Panel - System - <System Name> and with this modification the connection started to work when VPN using T-Mobile from anywhere. 

Thanks.

I'm assuming the name you are referring to is the name Windows displays by clicking Settings > System > Remote Desktop (with the text appearing):

"Use this PC name to connect from your device: yourhostname"

I'm wondering if this workaround is successful because for your situation, use of the pc's hostname causes circumvention of the VPN connection altogether.  If I'm right, you are instead connecting via another mechanism (such as SMB).  In any event, hostname doesn't work for me when I connect via cell phone using OpenVPN from either Android or iPhone.  For the Android Remote Desktop app the host's private IP address does work.  For the iPhone Remote Desktop app, it doesn't.

I used to setup the RDC to be established based on IP Addresses (11.56.36.9) but since it didn't work I tried by name. IE. jwindsor-dt and it works fine.

On my Macbook Pro, Tunnelblick was the issue, as soon as I moved to OpenVPN, the issue was resolved.

In relation to my iPhone, I set up another remote desktop with exactly the same settings and it worked perfectly.

I think if OpenVPN works, it maybe worth checking the Log File and see what it is connecting to.

Good luck

Just as a "FYI":  I'm not sure how the "DNS server workaround" mentioned above was implemented.  As another test, for "PC Name" in the RD Client, I replaced the private IP address of the Windows PC I am trying to connect to with my dynamic DNS server address but as I expected, I still could not connect via OpenVPN.  The reason I doubted it would work is that my dynamic DNS server address points to the internet address of my router (the same router that serves as my OpenVPN server) and not to the target PC with the private IP address.  As before, as soon as I switch from cellular to WiFi the connection works fine.