Hello,
I changed my router, I set the same SSID and password as the old router, I connected the old router, deleted the app, etc, none of them worked.
I have followed these steps a lot of times with the same result, it adds the sonoff switch but it shows offline, this is so frustrating.
Can someone please help me?
What you did is OK, you should be able to switch routers and by keeping same SSID and password everything has to switch and connect to the new router transparently. Perhaps you need to power cycle the devices so they do all the connection process again and get the IP address from the new router.
Check you have the 2,4Ghz band active in the new router, that you have set the DHCP service correctly and that you do not have any typo, extra character or space or case to uppercase mismatch in the SSID or password.