I started with a simple assumption: if I can access the router via web-browser, then I can also automate that. From that the proof-of-concept was headless Chrome in Docker and AI-directed code (code written via LLM, not using it all the time) that uses Selenium to navigate the code. This worked, but it internally hurt me to run 300MiB browser just to access like 200B of metrics every 10s or so. So from there we (me + codex) worked together towards reverse engineering their minimised JS and their funky encryption scheme, and it eventually worked (in the end it's just OpenSSL with some useless paddings here or there). Give it a shot, it's a fun day adventure. :)
Edit: that's the end result (kinda, I have whole infra around it, and another story with WiFi extender with another semi-broken different encryption scheme from the same provider) - https://imgur.com/a/VGbNmBp
You're correct, it gives you access to everything the Tether app can do.
I can't remember the details of the scheme, but it also allows you to authenticate using your TPLink cloud credential. If my memory is correct, the username is md5(tplink_account_email) and the password is the cloud account password. If you care, I can find my notes on that to confirm.