I’m paying 25€ for 600/300

One solution would be sending email requesting for them to open ports for you, you can tell them that you need open ports for security cameras and use that port for reverse proxy. That way you dont need more than one open port. Second option is cloudflare tunnels, they do not require open ports at all, but they do require for you to buy domain though. Last option would be to use VPN like tailscale which also do not require open ports at all. All you need is clients installed on devices and connect them on the same network.

Hope some of these help, i actually use all methods for different situations and each of them have pros and cons, but those are all reliable methods.