I don't use my "smart" 'phone for anything else but as a remote control for my Pentax cameras!
It's an old iPhone 6 I was given for the job and it has the latest o/s and latest version of ImageSync loaded, but no SIM card, so no other network influences!
Switch the camera on, start the wi-fi, switch on the 'phone and away it all goes … simply don't get any issues
However, if the camera is switched off, to change a lens or whatever, re-connection can be troublesome, taking a couple of attempts.
I've simply taken to switching the 'phone off at the same time as the camera and reconnecting from the start as necessary.
It may not be any quicker, but it's a lot less frustrating!
Inevitable, YMMV