Thanks for sending me the results of the tests. I am going to summarize (masking out your public IP info and full routes)
nslookup using your router default DNS
It is rejecting the local network ones - only allowing the public IP addresses. So yes this is DNS Rebinding protection in place in your router.
Your https route on internal network to your local IPs
> nslookup 10-1-1-6.ffxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx.plex.direct
*** Can't find address for server 10-1-1-6.ffxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx.plex.direct: No information
> nslookup 172-17-0-1.ffxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx.plex.direct
*** Can't find address for server 172-17-0-1.ffxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx.plex.direct: No information
Your https route through your public IP
> nslookup 210-24x-xxx-xxx.ffxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx.plex.direct
Server: 210-24x-xxx-xxx.ffxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx.plex.direct
Address: 210.24x.xxx.xxx
Your https route through Plex Relay IP
> nslookup 184-105-148-105.ffxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx.plex.direct
Server: 184-105-148-105.ffxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx.plex.direct
Address: 184.105.148.105
nslookups using Google DNS 8.8.8.8
The address lookups return the right IP addresses ok for local and public
> nslookup 10-1-1-6.ffxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx.plex.direct
Server: 10-1-1-6.ffxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx.plex.direct
Address: 10.1.1.6
> nslookup 172-17-0-1.ffxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx.plex.direct
Server: 172-17-0-1.ffxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx.plex.direct
Address: 172.17.0.1
> nslookup 210-24x-xxx-xxx.ffxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx.plex.direct
Server: 210-24x-xxx-xxx.ffxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx.plex.direct
Address: 210.24x.xxx.xxx
> nslookup 184-105-148-104.ffxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx.plex.direct
Server: 184-105-148-104.ffxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx.plex.direct
Address: 184.105.148.104
What this means is that local clients trying to connect securely using https then they will not be able to using the local IP route because of the DNS Rebinding Protection and so will go through the routes that work. That would be your public remote IP url above or if remote access is not working then the indirect Plex Relay route. The default is to force secure connections for all requests. Client apps have an optional setting for allowing fallback to insecure connections.
You could try and see if configuring DNS in the router to use Google DNS overcomes the issue.
Some routers do have configuring whitelists for DNS Rebinding Protection and one can add plex.direct. In my case I just live with the problem and make sure my Remote Access is always working - by adopting a manual port forward and manually specifying the public port rather than the unreliable uPnP automatic configuration
Noticed something strange in the server log and I am not sure what impact it has
When listing network interfaces, the loopback was listed twice - don’t know why that happens here
Sep 25, 2017 20:51:24.313 [0x2b0e073ef6c0] DEBUG - Network interfaces:
Sep 25, 2017 20:51:24.313 [0x2b0e073ef6c0] DEBUG - * 1 lo (127.0.0.1) (loopback: 1)
Sep 25, 2017 20:51:24.313 [0x2b0e073ef6c0] DEBUG - * 1 lo (127.0.0.1) (loopback: 1)
Sep 25, 2017 20:51:24.313 [0x2b0e073ef6c0] DEBUG - * 6 eth0 (10.1.1.6) (loopback: 0)
Sep 25, 2017 20:51:24.313 [0x2b0e073ef6c0] DEBUG - * 8 docker0 (172.17.0.1) (loopback: 0)
Sep 25, 2017 20:51:24.313 [0x2b0e073ef6c0] DEBUG - * 9 virbr0 (192.168.122.1) (loopback: 0)
See if you have any non standard settings
what does ifconfig show?
As mentioned before, there is a section on DNS Rebinding Protection here
https://support.plex.tv/hc/en-us/articles/206225077-How-to-Use-Secure-Server-Connections