I was searching the archives with regards sonoff switches failing to connect and showing offline. Been exhausting options until today I've cracked it so wanted to see if i can help anyone else! I could add mine to the app but they would show as offline.
Easiest fix as follows,
Split bands on router to 2.4ghz and 5ghz.
Buy a cheap and cheerful wireless n wifi repeater (with rj45 port)
Set repeater up on 2.4ghz channel in "AP" mode with Ethernet cable from router into booster.
Put phone with sonoff app onto 2.4ghz booster wifi connection.
Set up sonoff switches to connect through the new booster network.
Hey presto all working as it should whilst keeping standard 2.4ghz and 5ghz networks still available directly on the virgin router.
I really hope this saves someone some aggravation and countless evenings messing about like I did!
Ps. I did want to reply to an existing thread regards these issues but due to newcomer status not able!
Like you said, this is the case it seems alot of the time. By using a cheap and cheerful Wi-Fi booster as an access point running 2.4ghz only, it appears to have fixed all my previous connectivity problems with the router.