I have very similar setup, with ISP router that can’t be configured in bridged mode, so I have router after router setup (double NAT). Configuration I have is though slighlty different:
- on my ISP router I configured my router as DMZ. This way all traffic is by default redicter to my device and I do not need to take care about ports.
- on my router I configured redirection of ports 80 and 443 to SYnology NAS.
- on Synology NAS I configured reverse proxy, but here is major difference: I use subdomain names to redirect different services to appropriate appliances on my network. E.g https://hassio.mydomain.com is redirected to internal IP of my HA on port 8213.
I’m not sure if redirection you try to do on Synology will work. I think myname.synology.me is always pointing to DSM itself and accepts connections on port 5000/5001 (unless changed). The whole purpose of reverse proxy is to use subdomain names rather, to identify required redirection, than specific ports. Some details of my config are here.