Get online support
Hi @ciciliu314
Welcome to the Community!
Have you tried using a simple password? For example, 12345678? If not, please try that first.
If it still did not work, then you may reset the modem/router to factory default. Once done, you may need to reconfigure the settings that you've already personalised.
If you are still not able to connect to the 5GHz network even after the reset, please let us know.
BasilDV