6in4 tunnel, proto-41 connection unanswered

Hello,

I’m trying to establish a 6in4 tunnel with tunnelbroker.net.
I followed the instructions in their website for the usage with RouterOS, however, while I can successfully ping the 6in4 server and he can ping me, the interface cannot connect and in the IP/Firewall/Connections I can see the connection as Unanswered.
I’ve added an input and an output filter to accept proto-41: in the input filter I have 0 packets, while in the output filter I have a slowly growing packet count.
I am using a RB750GL (RouterOS 5.16) connected to a DrayTek Vigor 120v2 DSL modem. The Vigor is set in bridge mode and I have a static IP assigned to the RB750GL ether1-gateway interface; the RB750GL has NAT masquerade enabled on ether1-gateway interface and is serving private IPs to ether2-master-local attached devices.
The strange thing is that if I use my Cisco E4200 instead of the RB750GL, the E4200 can establish a connection (but apparently after the 8h IP lease expires, it doesn’t auto renew it, so I’m trying to use the 750GL to avoid this).

Thank you