Your environment is a little different than I am used to, but the same principles apply.
In simple terms Serviio advertises itself to all devices on your network, and these devices then respond to let Serviio know that they are available. Normally your router will stop this advertising going out to your service providers network. I'm not an IP specialist (others on the forum maybe) but you will need to look at your router to determine if it is allowing these broadcasts through - check UDP Port 1900 for starters.
Your case could be that your service provider has you traversing their LAN, probably through a DMZ which contains an Internet proxy and other devices, in which case there may not be anything you can do without their involvement.
Alternatively, you could simply right click on the device in Serviio console, and untick the "Device Allowed" - while this will deny that device access to your media via Serviio it doesn't address the broader issue - you do not appear to be isolated from your Service Provider's network. Also, if that devices' IP address changes, you will need to re-do this.
DenyAll wrote:You may need to follow this up with your service provider
I meant you may need to discuss this issue with your service provider. This is still true.