@F-K 

It is okay to use two phones to manage the same router via Tether App at different times, but not possible to login at the same time.

Normally a second login will cause the first one logged out automatically, but you should still be able to login again on the first device after that.

Since you mentioned that the error message is incorrect password, it is recommended to double check the login password and ensure it is the same password as the one used to login the web interface.

By the way, are you able to login the router via Tether App on second phone now? If so, please check if 2FA is enabled on the Tether App, if so, check if both phones are trusted device or not: Introducing Two-Factor Authentication (2FA) on Tether App