Apologies I've been a bit slower than ideal posting on the forums lately. I try to reply ASAP, although although I'm pretty busy behind the scenes. As much has possible I try to put some time aside to catch up on the forums. Things have been particularly crazy lately and every time I've planned to drop in and answer questions such as yours, I've been interrupted with urgent stuff I've needed to take care of.

Anyway, glad to hear you worked it out.

FWIW a 502 is always an issue connecting to the server. All 50x errors are something behind the front end server. The most common causes are a reverse proxy not being able to connect to the background server (like changed server IP in your case). The other common cause is if something has crashed on the serve itself. Although because we use Apache with mod_php and Apache is some reliable, getting a 502 with a PHP app (like WordPress) are pretty uncommon.

For future use (and anyone else stumbling across this post). The first thing to check with a LAMP based TurnKey server is log in and check that Apache is running:

systemctl status apache2

If all is well, the line starting "Active" should say "active (running)". The next thing to check is that it is listening on the appropriate port/s. Note that some services (such as Apache) only note IPv6, but they are actually listening on IPv4 too. So if Apache is listening publicly on ports 80 (http) and 443 (https) as it should be you'll see those lines in the netstat output:

tcp6       0      0 :::443                  :::*                    LISTEN      4187/apache2        
tcp6       0      0 :::80                   :::*                    LISTEN      4187/apache2 

If both of those look good, then it's almost certainly something external to your server.

Good luck with it all.