The DSL port will NOT work if you install OpenWrt onto the D7800.
OpenWrt does apparently work for the rest of the D7800 hardware, so it can be used as a regular router (without modem).
I added the following to the wiki page last night:
Important: The DSL modem within the D7800 is NOT supported by OpenWrt.
after searching the forum to confirm my suspicions there is absolutely no support for the little known Lantiq VRX320 modem chip.
Netgear Nighthawk D7800 vs R7800
If for example you wish to use a D7800 with OpenWrt as the primary router wired to a VDSL/FTTC line, you will need to install a 5-10 years old Openreach (Huawei or ECI) VDSL modem or similar device.