My iPhone and iPad have suddenly stopped connecting to my apple router?

My iPhone and iPad have all of sudden started not connecting to my apple router; but they connect fine on the guest n/w I've created on the same router? I've restated the router, changed channels but I'm now a bit lost to understand why after years of use and no problems why devices are now struglging to connect. The wireless network is there it just seems these particluar devices can't now connect. I've tried "forget this network" but nothing seems to work - so any help and suggestions welcome - thanks.