I have just recently get Plex for iOs. I have problems using it (for ipad and iphone) using my local wifi, meanwhile it is working properly using 4G. I use WIFi connection in those devices for others application working normally and i do not know if there any kind of configuration that are required for this purpose. Thanks in advance.
What is your vendor of your wifi?
Do you have any wifi guest mode setup on that thing?