Maintenance for the week of April 20:
• NA megaservers for patch maintenance – April 20, 3:00AM EDT (7:00 UTC) - 12:00PM EDT (16:00 UTC)
• EU megaservers for patch maintenance – April 20, 7:00 UTC (3:00AM EDT) - 17:00 UTC (12:00PM EDT)
I just tried it with my NordVPN and didn't have any issues connecting or signing in. This may be specific to your VPN and I would try a different VPN server if possible.
Unfortunately people who wish to be nefarious will often use VPN's in order to stay anonymous as to their actual IP address and that means companies will often ban the IP addresses of the VPN provider.
I needed to use a VPN for almost a year to play LotRO. My IP was being prevented from accessing their servers about 2 steps before the LotRO servers. No idea why, SSG said my account was fine, ISP said everything was cool with them, but I just couldn't get through. The VPN worked.
The problem simply vanished after myself and several others took to the forums over there to ask wth was happening. I think SSG either fixed something or spoke to one of the in-between server providers.
That's weird, I actually HAD to use express VPN to play over my phone tether about six months ago. I haven't had to do that in a while so I can't say whether it still works for me or not (don't have it on my phone anymore to test, sorry) but I know it worked perfectly fine for me and was, in fact, the only way I could even get the game to load. Otherwise, I would just get kicked back to the login screen.