The only fiddly part of the setup is creating the VLAN and assigning it to the WAN interface. I used PFsense but the settings will likely be identical for Opnsense. This is how to do it on PFsense.
https://docs.netgate.com/pfsense/en/latest/vlan/configuration.html
So follow the above guide except put in 911 as opposed to 10 at this step " The VLAN ID number, in this case, 10"
Personally, I switched from PFsense to IPfire which is so much easier to get the basics setup.