Actually the note wants you to change the REMOTE management port (Service port) but not the internal port. The internal port 80 is reserved for the local login of the router.
And I guess 10.0.0.1 is the LAN IP address of your router, right?

In this case, if you want to use port 80 for your local device, the correspondences must be:
router management page: 8080 (service port) <-> 80 (internal port)
service on local device: 80(service port) <-> suppose it's 88 (internal port)

suppose the LAN IP address of the router is 10.0.0.1, the WAN IP address is 1.1.1.1, the IP address of the local device is 10.0.0.10.
Then you can use different methods to access the management page and the service on the local device.

Access from internet:
management page: 1.1.1.1:8080
service on local device: 1.1.1.1:80

Access in local network of the router:
management page: 1.1.1.1:8080 or 10.0.0.1:80
service on local device: 1.1.1.1:80 or 10.0.0.10:88

Thanks for the quick reply!

I already throught about the fact that the ports could be reserved.

Would there be a possibility to change that behaviour? Or are there any plans to add an option for changing those ports?

At least to allow port 443 if port 80 is reserved for local login?
On the public IP is a website running, so the default HTTP(S) ports are needed.

edit: port 443 works well, so only port 80 is reserved. this is not perfect (for visitors not accessing https directly but http) but quite ok.