First, you will have to navigate to /etc/config folder:
Then edit file uhttpd:
To start editing the file press letter " I " on the keyboard.
Find option called:
option redirect_https '1'
Edit this option to:
option redirect_https '0'
Save it by pressing ESC on your keyboard and writing:
Press enter and you should have "Redirect to HTTPS" disabled.
It can take time for config to reload, but if it doesn't save you can just restart your router and it should be not redirecting to HTTPS anymore.