Do you have experience with the Cisco SG250X-24P-K9-EU switch?

By the way, double NAT is not a Ubiquiti problem. You always have that when you have two routers connected in sequence. So if you only use one Ubiquiti switch with a Fritzbox, you don’t have that issue at all. The problem would actually be solved by using a pure modem in front of a router (in UBNT's case, the USG).

Everything is clear to me. As far as I know, the Ubiquiti switch is not an L3 switch.

Edit: just saw, the Pro apparently is...
Edit2: and damn expensive

So DSL connection -> modem -> USG -> switch -> Fritzbox for telephone?

I understand everything. To my knowledge, however, the Ubiquiti switch is not an L3 switch.

No, but that wouldn’t matter if you only operated a modem in front of the USG. Then you would only have the 1 GBit bottleneck if you have a lot of traffic between the VLANs.

So DSL connection -> modem -> USG -> switch -> Fritzbox for telephone?

Correct.

Which Ubiquiti switch are you talking about? There are different models. I have two ES-24-250 Watt and they also support Layer 3 routing.

As I said, what really puts me off is the tinkering with Magenta TV. Of course, there are solutions, but no one seems really happy with them.