It’s pretty normal for passwords to be “plaintext” inside an HTTPS request. That’s how practically every login to a web app works. If it’s not HTTPS, there’s a whole slew of other issues along with putting a plaintext password in the request.
If it is HTTPS, then the issue really is just that the password gets sent anywhere rather than staying local. This is a lot more debatable as a practice, but unfortunately is also common for a lot of routers to support their cloud/app management functionalities.