Hello,
Support for longer usrenames from the WebUI side will be included in 7.5 firmware version.
At the moment, longer usernames can be edited via CLI/SSH.
If you want to edit username length of the root group user for WebUI login, execute the following commands:
- uci set [email protected][-1].username='newuserlonger'
- uci commit rpcd
- /etc/init.d/rpcd restart
For users of other groups, you will need to edit rpcd configuration file. First, execute the following command:
Press letter i to enable editing and edit username option under login section for a particular user:

Once done press Esc, then enter :wq and press Enter.
Now, you should be able to login using longer usernames.
Best regards,