Problem: Pirelli modem is not transparently routing traffic for tcp port 7070 and 554.
I’m using a Pirelly VDSL modem at home in pure routing mode (no NAT, no QoS). When starting a TCP connection to port 7070 (or 554), the Pirelli modem answers the TCP SYN with SYNACK, then it tries to connect the original server with the modem outside IP, when the server rejects the TCP connection, the Pirelli modem closes the client connection with FIN.
It seems the Pirelli modem has some kind of transparent proxy for these ports. But I didn’t turn that on and don’t know how to turn that off.
I tried to fix this with an update but it seems the software is provider specific (4.8.3.PRGAV_SIL_18.104.22.16807).
Solution: I think I have to remove this Pirelli modem, and by a small Cisco router.