Why can't the web app find my Plex Server?

[SERVIDOR_1]

Server Version#: 1.32.5.7349
Web Player Version#: 4.108.0

I have a strange problem and I don’t know when it began. When I am home (in the same network as my server) I was able to access my Plex server from a Windows web browser by going to https://app.plex.tv and also by going to https://192.168.1.5:32400 which is my internal address. Either one of those would work. However, in the past few months, when I am home, I can only reach my server if I go to https://192.168.1.5:32400 on my web browser. If I attempt to reach the server via https://app.plex.tv it refuses to connect saying that my Plex server is unavailable. Interestingly, I have no issues reaching my server from the outside world. This only happens when I am home.

In addition, when I connect using https://192.168.1.5:32400, a notice appears saying that my connection is not secure. Something about the certificate. Then, after bypassing the warning on the page, I am able to gracefully connect and my Plex server content loads fine, except that https is crossed out in the URL bar, indicating that the connection is not secured. And once again, if I was to attempt connection using https://app.plex.tv, some of the page components load, but complains that my server is unreachable.

The only times https://app.plex.tv work fine is if I am outside my network, somewhere else on the internet, or if I use a VPN, then it all works great. However, at home, the https://app.plex.tv address does not work.

I have have followed all instructions in the troubleshooting document at: Why can’t the Plex app find or connect to my Plex Media Server? | Plex Support

I have tried using another computer, other browsers, and have the same exact result. I use Windows Defender and do not use any other firewall software or antivirus. Also, the Secure Connections settings in my server is set to Preferred.

It is worth noting that mobile devices, such as iPhones, iPads, and Android devices, have no trouble whatsoever connecting to my Plex server using the Plex app whether I am home using my home WiFi, or from an external or cellular network. The problem I described only happens using any web browser in either Mac, Windows, iOS, Android, as long as I am connecting to my home WiFi and without a VPN. I have tried Firefox, Chrome and Safari and all exhibit the same exact trouble. Once again, this problem happens in web browsers only, and does not happen when using any of the Plex apps.

This seems to be an issue either with my server or with my router. I am highly technical in these topics, but here I am at a loss.

[SERVIDOR_1]

Ok I feel silly posting the solution within minutes of posting the question, but I hope that this could help someone else in the future.

After reading some more into this, the issue I had was related to “DNS Rebind Protection” which is a security feature built into my router. I disabled the feature and then Plex web access began to function as expected regardless of URL used. Plex also has a related feature called “Treat WAN IP As LAN Bandwidth” in the network settings menu. However, in my attempts to troubleshoot this issue, checking or unchecking that option made no difference. It was only when I disabled DNS Binding Protection in my router that things began to work as they should.

To make matters more interesting, after verifying that everything was ok, I went back to my router configuration and re-enabled DNS Binding Protection and the problem did not return! I also unchecked and checked the “Treat WAN IP As LAN Bandwidth” setting in Plex server but it made no difference and things continued to work regardless. I made several attempts, closing the browser, and reloading Plex.tv a couple of times, and it seems that now the problem is resolved. I can only think this might’ve been a bug in my router’s software because after re-enabling DNS Binding Protection, things continue to work just fine.

[SERVIDOR_1]

So the problem returned. Took over an hour for it to start happening again. So instead of disabling DNS Rebinding Protection altogether in the router, I created an exception for my Plex server so it is the only device that is not under DNS Rebinding Protection. Immediately after I did this, web access locally began to work normally.