Well, it was a nice try to change the router. From what you say it is the same model router, but at this point I don’t think that’s it. The vintage of your IoTaWatt is 2017. The newer units use a later model ESP12S and although there have never been reports of problems like this before, my gut says it’s the ESP WiFi.
If you PM your email and shipping address, I’ll send a replacement PCB. you should be able to just swap it in with your SD card from the old PCB and retain all of your configuration and history.
If that doesn’t solve the problem, we would need to do some WireShark to flesh out the problem. You sound like you may be able to do that.