I would like to configure nat loopback on a GL-MT3000 while operating as a repeater so that I can access my local nextcloud server using the public domain name from inside the local network.
Unfortunately, I don’t really understand what you want to achieve.
Why NAT and what does “public domain name from inside the local network” mean?
Normally you would arrange this via a different DNS entry for internal systems.
I have registered for a dynamic DNS service so that I can access from anywhere my NextCloud sever I manage at home. Let’s say the domain name is: www.myncserver.com.
I am able to access the server by the domain name without any issues while on any network except my local one where www.myncserver.com is running.
What I’m trying to accomplished is known as the following but understand that there might be additional terminology:
NAT loopback
NAT hairpinning
NAT U-turn
I have been able to accomplish what I need while configuring the GL-MT3000 in router mode and adding a DNS entry but the same procedure does not work while configured as a repeater.
I’m not very familiar with NAT, as I’m always happy if you only need a destination NAT and don’t have to touch anything else. No routing is done in repeater mode, so - as far as I can tell - NAT is not used either.
I would therefore realize your goal differently: The currently addressed DNS server (usually the main router) should simply respond to the DDNS address with the internal IP address. Whether and how this works depends on your router.
I would assume that this will not work anyway, as the NAT rules would then have to be executed on the ARRIS BGW210-700. And this probably does not offer this.
A quick solution would be to use ZeroTier (https://www.zerotier.com/) on all devices. ZeroTier then connects within the network without using the Internet - and outside the network via the Internet. That way it would always work.
ZeroTier is really easy to use, you will be able to use it quickly. On the NextCloud server it is perhaps a little more difficult - but if Linux is used as the OS, this will also work after a short time.