Hmm I’m not sure if I can help you in this case.
The only thing I remember is that I once had a problem connecting I think S26R2 to a certain router and regardless of the settings I couldn’t maintain a stable connection.
Changing to another one helped… I don’t use tp-link so it’s hard for me to guess… from what I remember I had some problems connecting some sonoffs to OpenWrt but here the problem was solved by using the option “Allow legacy 802.11b rates” but I suspect that tp-link may not have a similar one.
Of course make sure that your sonoff has correctly saved the data of the wifi network to which it should connect. If the router has logs, check if any connection is taking place.
If it connects to the router, also check if the device is receiving IP addressing from dhcp.