Could be related to DNS Rebinding.
To check, have a read of this post in the thread Remote access failure to identify your local plex.direct hostname (it will be the in the format https://AAA-BBB-CCC-DDD.HHHHHHHH.plex.direct:32400 and have the status local="1".
Copy the address corresponding to AAA-BBB-CCC-DDD.HHHHHHHH.plex.direct. Open a command prompt on your laptop and type the command nslookup and paste the address there (i.e. nslookup AAA-BBB-CCC-DDD.HHHHHHHH.plex.direct.
If it returns the IP address as AAA.BBB.CCC.DDD to match the plex.direct local name, and this also corresponds to the local address of your plex server then it means that DNS Rebinding is unlikely to be the issue. If it returns a different address then it’s likely to be a DNS Rebiding issue.
As you don’t have access to the router you could manually enter a different DNS server on your tablet or phone.
