Thanks for testing that.
Uptime Robot test to your public IP address and manual port. It shows your server is reachable, so that indicates your server config and router config are working correctly. Also, that your server is reachable from Internet.
What Uptime Robot does not test is your server registering with the servers at Plex, so it can be found by remote clients. I can see the registration changing in the log file (up/down), but I cannot tell if it is the registration failing or you testing things (a comment on my log reading skills, or lack thereof).
The only other suggestion I have is to upgrade to the latest release instead of staying on 1.18.5.2309, which is a year old (released Jan 2020).
Plex made changes in the 1.19.x releases (April/May/June 2020) which improve the reliability of the status indicator on the Settings -> Remote Access page.
The Plex Media Server would time out when trying to contact the hosts at plex.tv. As a result, the Remote Access settings page shows service as unavailable when it is still available. Plex made changes to PMS and on the back end servers to remedy the situation.
Links to release announcement thread:
1.19.2.2737
1.19.4.2854
1.19.4.2935
