"Couldn't connect to relay" when trying to use Steam Remote Play via the Android Steam Link app when on separate networks

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I'm having trouble using the Steam Link App and Steam Remote Play to play games on my Android (Samsung A35). I've unlinked and linked the devices multiple times, restarted Steam multiple times, and disabled and enabled Steam Remote Play multiple times. The TCP and UDP ports are allowed through my Windows Defender Firewall. I have no other Firewalls. I've restarted my PC. Nothing works and nothing resolves the issue.

It works perfectly fine when my phone is on the same network as my PC, but once I switch to a different network or to 5G it stops working. Big Picture mode activates, and looking at the Steam Console shows that the connection is seen, but my phone gives me the error "Streaming Error - Couldn't connect to relay 155.133.238.xxx:3478" and the Steam Console gives me the error "Timed out attempting to negotiate rendezvous". I've changed the last bit of the IP to xxx since the specific subnet seems to vary between 179 and 195.

Attempting to ping the relay server via "ping -t" in the command prompt fails, though that might just be the due to the server's firewall. Regardless, my suspicion is that the relay server my Steam Link is attempting to connect to is offline, misconfigured, or broken. Or am I missing something or doing something wrong?

My PC:
* Windows 10
* The firewall is definitely not the problem since the handshake succeeds and Big Picture launches. I'm also only using Windows Defender set to minimum and the UDP and TCP ports for Remote Play are explicitly open
* I'm on a 500 Mbps wired symmetrical network that has never given me any issues

My Phone:
* The latest version of Android running on a Samsung A35
* My 5G connection is stable, as are the other networks I've tested this with
* Testing the connection to the PC before attempting to connect results in Steam Link saying the connection is good

What's Happening:
* If my phone and my PC are on the same network, I can use Steam Link on my phone to use Steam Remote Play without any issue
* If I disable my WiFi and switch to 5G on my phone, Steam Link sees my PC and considers the connection good. When trying to connect to Steam Remote Play, Big Picture mode launches on my PC. Despite Big Picture launching, I inevitably get the error "Couldn't connect to relay 155.133.238.xxx:3478" on my phone's Steam Link

What I've ruled out:
* It's definitely not my Firewall: being on the same network works fine, and when not on the same network Steam Big Picture mode launches even if the connection fails in my phone's Steam Link. There are no errors on my PC, the connection is recognised, and everything is clearly allowed through. My Firewall is also incapable of interfering with my current settings, and for safety I've allowed the TCP and UDP ports explicitly.
* It's definitely not a linking issue: I can play perfectly fine when on the same network. My phone is paired in Steam Remote Play. When not on the same network, Steam Link still recognises my PC and sees it's online, ready, and with a good connection
* It's definitely not a network issue: My PC network has fantastic upload and download speeds, and on my phone Steam Link sees the 5G connection as 50 Mbps and says the connection is good. When I try to launch the Remote Play session, Steam on my PC launches Big Picture mode in preparation.

My suspicion:
* The relay server (155.133.238.xxx) has an incoming port 3478 (likely both UDP and TCP) and outgoing ports UDP 27031 and 27036 and TCP 27036 and 27037.
* My phone's Steam Link starts a handshake with my PC's Steam. The handshake is verified and Steam Big Picture launches. This handshake uses the TCP 3478 port.
* With the connection between my phone's Steam Link and my PC's Steam complete, my phone attempts to connect to the relay server via the UDP 3478 port.
* For one reason or another, my phone is unable to connect to the relay server. Either because the relay server's firewall is incorrectly set up, UDP port 3478 is blocked, or because of software issue on the relay server.
* The relay server is not used when on the same network as my phone and my PC can be connected directly. Because a connection to the relay server is not required, the issue goes unnoticed.

I've contacted Steam support and the only replies I get are canned responses, "We don't provide in-depth technical support for Remote Play", and an insistence that I turn to these discussion forums for a solution. Despite multiple people having this issue, Steam still refuses to offer proper support and instead wants their community to fix it.