If you’re starting the mediabox setup on the isp network, it’s doing local natting with iptables, based on the IP that it resolves from the hostname. Probably would need to shut down and re-up to walk between the deco’s and the isp wifi domains.
I agree with the other comments, looks like you might be in a double NAT scenario - fortunately for you, I think I know how to fix it, seeing as we’re both running deco’s!
You want to go into the smartphone app, go to ‘More’ at the bottom right, (as opposed to ‘Network’), Advanced > Operation Mode > Access point.
Be aware this will cause a disruption, and anything connected to them will need to be reconnected so it gets dhcp/ip addressing from the isp router rather than the deco.
The other alternative is, if they’re already in AP mode, it might be recognizing the deco SSID as a separate network to your ISP’s router, and randomizing your mac address (for anonymity across airports and hotels and such). Then, with your original mac address holding the first IP in lease, your ‘new’ mac address gets a different one. Check your mac with ip link too when connected to the two different networks, and see if you can find an option to set it manually for both networks, or just use your default one for those networks.
I’d love to hear how you get on, I’ve been putting off building this exact solution (mediabox) from scratch, had no idea there was a project set up to run it all