The proper setting is 115200 N-8-1. You are seeing blocks because the 8th bit reads 1 when it should be zero. Setting parity basically ignores this bit, but there are still lots of errors in the other bits occurring.
Check your ground wire. Don't power the unit from a USB port on the PC, use a separate power supply that doesn't have a common ground.
I don't think this model requires resistors to be added to make the serial port work but many TP-Link do.
I used an Arduino with built-in USB-serial by unplugging the uP on the first device I wanted a serial connection to, and it worked fine.
A couple of years later on a different device I got the same results as you. After fruitless attempts with pull-hi/-lo resistors, etc I got one of these old Nokia phone serial cables - the blue ones work AIUI, but not similar black ones: https://www.ebay.co.uk/itm/174777286684.
The pin-outs are easily found online. After cutting off the phone end and using an old PC CDROM drive cable to get female pin connectors to solder in on the GND, TX and RX lines I had a perfect serial session .