If the DHCP service was running on the Asus Router, then yes, the reset of that router to defaults would have wiped the IP tables in use I'm afraid. However, since this Asus Router was connecting to the Virgin Modem/Router, there's a good chance that the Virgin Modem/Router was the one handling DHCP and have that info stored there (assuming it's still physically there for you to access and it also hasn't been reset to defaults).
Otherwise, you'll just need to plug in the Plex server and then try and access via the IP it gets there locally if possible.