I uploaded the relevant openwrt firmware to my D7800 and now I cannot access my router. I have done the following to try and resolve.
Tried to get the router back to factor settings by
Rebooted D7800 holding down the reset switch for 15 seconds. Only the power led activates
Rebooted D7800 holding the reset prior to powering up and waiting for the power light to flash before release. Only the power light activates
As I cannot access the WiFi on the router I cannot access the gui.
While the d7800 and r7800 differ in quite significant ways (ipq8064 vs ipq8065, modem vs no modem), the recovery procedures should be very similar, so check the r7800's device page https://openwrt.org/toh/netgear/r7800#debricking (obviously using the d7800 images instead). Both devices should be relatively safe against (hard-)bricking, so push-button recovery should help you out.
Unfortunately you didn't specify how you ended up in that kind of a pickle, I assume you flashed 18.06.x or newer either from the OEM firmware or an OpenWrt/ LEDE release predating 18.06.0 via their webinterfaces. Given that the partitioning changed between LEDE 17.01.x and OpenWrt 18.06.x this will result in a (soft-)bricked device, you do need to use push-button tftp to change between OpenWrt >=18.06.x on one hand and (OEM || LEDE 17.01.x) on the other, please see this for details: