In reply to a post by RichTea23:

Zen have a custom image on the FRITZ!Box 's they provide so will have your data in already

IIRC Zen use TR-069 autoprovisioning, the custom image contains default PPPoE credentials plus whatever URI/certificates/credentials they use for TR-069. When first connected the FRITZ!Box will establish a PPPoE connection using the default credentials, connects to the TR-069 server and reports various data including the device serial number. The server looks up the user account based on the serial number and reconfigures the FRITZ!Box with the user-specific credentials, plus any additional settings such as DV if the user has opted for it.

Just to be clear, what exactly have you tried from that post?

Cityfiber based FTTP connections use VLAN ID 911 (I presume thats the only one they use?!) so it is essential that you apply a VLAN to the WAN port that connects to the ONT.

And then apply the PPPoE connection to that VLAN, not the WAN port.
I'm using OPNSense (and Giganet not Zen) but there should be little difference given that OPNSense was forked from pfSense and both ISP connections operating over cityfibre.

Physical WAN Port -> VLAN 991 -> PPoE is the key, or in OPNSense terms...

So basically, for IPv4 (lets forget ipv6 till you have v4 working!)

I have a main external OPNSense interface called WAN (its just a name), this interface's (parent) "device" is PPPoE (ip4v type PPPoE) in my case its called/labeled "ppoe3" and has the pppoe credentials applied.
: : :
pppoe3's interface's link type is PPPoE and link interface (parent) is a VLAN named igbe1_vlan911
: : :
igbe1_vlan911 is a VLAN I/F with VLAN TAG (ID) 911. It's parent i/f is igb1
: : :
igb1 is the physical port on my Protcli VP2402 that is connected to the ONT via ethernet cable.

Of course the pfSense terminology may be slightly different but it should work - I did have it working before I switch to OPNSense which I have found superior and updated more frequently than pfSense.

Thank you very much for your help I have finally managed to get it to work. Well, it was a bit more challenging than I thought originally and it was well above my level of understanding of network topology.
But following your lead I created a VLAN in pfsense with TAG ID "911". Then under Interfaces/PPPs section I created PPP interface with VLAN port and supplied user name and password. Next under Interfaces/Interface Assignments section I assigned that PPP interface to pfSesnse WAN. Connected my router to the Zen box on the wall and it all worked as expected.
Once again thanks for you help

Tom