VPN fails to connect with UDP, OK with TCP (NordVPN)

[#1]

Windows client and Android client both fail to connect to VPN server (NordVPN) using UDP over the Plusnet network. Both connect OK using TCP and both connect OK using either protocol over the mobile phone network. So the problem is only with Plusnet and UDP on port 1194.

I have tried to diagnose the problem using tracepath and nc, but the results have been inconsistent. A similar problem was reported here:

https://community.plus.net/t5/Fibre-Broadband/OpenVPN-UDP-fails-to-connect-TCP-works-fine/m-p/147711...

but the reported fix, reducing the MTU, did not work for me. This set-up worked OK until a few weeks ago and initially I thought the problem was a new version of the NordVPN client, but reverting to an older version didn't help. In any case, the logs indicate a network connection error. Turning off the router and PC firewalls did not help.

Ideas anyone?

[#2]

Moderator's note:

Moved from Broadband.

[#3]

I fail to understand why this has been moved to "Tech Help - Software/Hardware etc". This is clearly a problem with the Plusnet network.

[#4]

I don't have an issue with connecting to PIA VPN over UDP.

[#5]

This appears to have been resolved by factory resetting the router.

See https://community.plus.net/t5/Broadband/DNS-servers-unreachable/m-p/1531950#M322959

which seems to be a related problem. I have installed a spare router and will monitor the situation over the next few days.

[#6]

The problem returned, even with a different router. I'm convinced now that this is related to to my DNS problems - DNS requests and responses also use UDP.

I will continue to update the DNS thread, rather than this one.