I don’t know how Entware works, but probably it installed rsync not in /usr/bin, but somewhere else. Find it somehow (e.g. by running find / -name rsync).

rsync has a command line option --rsync-path.

I’d guess that the reverse proxy would run on a cheap VPS with an IPv4, and connect to your home through a VPN like wireshark.

I think your understanding is mostly correct. But I never heard that you can get an ISP to port-forward through their CGNAT. Either you get a public IPv4 address or not.

IPv6 would work, but then only clients with IPv6 can connect. And I just read that there are still people with IPv4-only routers in 2023.

Have a look at Tailscale and Zerotier, I think they are often used to poke through CGNAT.

From your Linux systems you could run grub-set-default or grub-reboot. I don’t know if there’s a version for Windows.

I don’t know how it works with your or OP’s router, but my router has a firewall for IPv6, too. There’s no NAT for IPv6, so if I open a port I have to use the server’s IP address, and that’s also the IP address that I have to use from the outside.

At least Android devices are not firewalled in any way. Even with the latest Android 14 I can run servers like ssh/ftp/ScreenStream locally on the phone.

There’s a firewall/NAT on the phone network, but in the local network it’s perfectly possible to connect to other phones (unless the local network has client isolation).

Check if your Provider uses CGNAT. And I don’t understand why you opened port 22-29 instead of just one port.

What url are you using on the phone? localhost is the local host, so your phone in this case.