How can a TC disk be seen by upnp clients?

If your "modem" supports UPnP, then it is not just a simple modem, but a combination modem and router or gateway device. I would suggest that you enable UPnP on your modem and reconfigure your TC to perform as a bridge. UPnP "automatically" enables opening/closing ports on NAT services. With the TC in bridge mode, its NAT services will be disabled and your "modem" will be the device running these services for UPnP to administer.

I know that upnp and dlna are not supported by apple but there must be a way.

The issue is access to the hard disk via UPNP must be on the device itself.

Turning on upnp on the modem.. is not going to work. Nor are opening ports. UPNP is designed primarily for opening ports to WAN .. no such tool is needed for LAN where by default all ports are available. But it serves other purposes for access on the local network.

The disk of the TC is perfectly seen by my WD live TV via wifi

Because the WD live can read and access the disk over a whole variety of protocols.

Apple deliberately restrict the airport access. I don't think you can work around it.

If your modem router has a USB port and is capable of plugging in a USB drive.. that would work.

Or simply buy a cheap NAS. Search for a second hand one or two drive slot one that someone is upgrading. The better brands like QNAP and Synology are recommended.. but I have a cheapie made by Zyxel and it works fine because it fully supports upnp and dlna.

Windows can access the TC disk. That is over standard SMB networking.

The two protocols apple provided.. AFS and SMB1

The problem is on any new windows system.. SMB1 is turned off because it is a security problem. Recent ransomware attack used the weakness of SMB1. And windows cannot use AFS.

The WD live is using SMB1.. but since it is a dedicated device the security weakness is not such a major issue.

Looking at the specs for the stream magic v6 it can indeed accept a digital input so you should be able to put the WD live in front of it and stream through it.. very clumsy though.

I suggested earlier that you can plug a hard disk into your router.. do you know you can plug a hard disk directly into the Stream Magic via USB? You will not have speed problems using USB directly which you do experience over network.

that will solve eveything if I don’t consider the money spent on the Tc 😠

Remember Apple build their products around a certain model.. in the case of streaming music that is 100% total and absolute committment to iTunes and nothing else. The Apple routers are built without media extensions. It has no place in the Apple architecture for how things work. DLNA and UPNP are not supported protocols in Apple devices. They are in competition to iTunes and airplay. Apple will not support them.

A 2009 TC is ancient now.. it is frankly a miracle it still works.. most died at 18months after purchase due to poor ventilation and overheating power supplies. Really if it lasted 3 years which is design expection you got your money's worth.. at least as far as Apple is concerned.