"Cannot Verify Server Identity" - Cannot find source
Look in the settings app, delete all profiles you don’t know you need and maybe the rest. 

Also, VPN profiles could be implicated and for an edge case, look to any iCloud keychains on macOS. 

I would make a back up as well in case you end up erasing the contents and settings and restoring from backup after being sure it’s not coming from your network with a clean iOS device that joins your network.