My configuration prior to upgrade used port 8000 to access the router. There was a bug in the firmware such that they were using port 8000 themselves in the firmware for some other function, so it caused me to not be able to login to the router. That allowed tech support to remote in and use the command line interface to change the web interface access port from my setting of 8000 back to 80. I was then able to login again. I have subsequently changed my access port to 8080 and it is working fine.
They said they planned to fix this issue in a future firmware release, but frankly I have not tested if they fixed it since all is working well for the web interface on port 8080.
I don’t know if this will address your situation, but I hope it helps. If your problem is similar, you may have to revert to firmware 6.x to make the change then reboot under firmware 7.x, especially if you don’t have remote access enabled for tech support to be able to remote into your unit and to use the command line interface to fix it for you under firmware 7.x.