I already posted this solution a year ago and it still works :
1. Create a guess ssid/password on your router/access point with no special characters such as abcdefgh/12345678 and connect your phone/tablet to it
2. Add a new device in eWeLink in the usual manner (5 seconds + 5 seconds depress of the button on the Sonoff and Quick Pairing)
3. Once done, upgrade the Sonoff to the latest firmware !
4. Then you can reconnect your phone/tablet to your normal ssid/password (even containing special characters) and redo step 2 above again. Voilà. Works every time for me with different types of Sonoff's and routers.
The issue seems to be that old Sonoff firmware cannot cope with recent routers config/security. I think I have read most of the posts for the last 2 years and the many other suggestions, sometimes ingenious and exotic, do not seem to make any difference for me and therefore appear to be superfluous :
- creating a new eWeLink account
- deleting the device before re-adding it
So remember, upgrade your Sonoff's regularly and definitely before replacing an old router!
An alternative to setting up a guest ssid if you do not want to mess up with your router is to set up a phone in tethering mode with a simple ssid/password (see step 1 above), then connect your tablet to that ssid, then go back to step 2-3-4.
HOWEVER: I just tried my trick with a Blue50 (WL SW01 10) WiFi Smart Switch I once bought and forgot about! And it does not work! I had even resigned myself to tosmota-ing it as suggested in one of the posts (sorry, I do not recall who) but the Tx/Rx are not exposed. So that £4 gadget will go in the bin. Should have stuck to Sonoff, better devil you know !