FWIW, I contacted someone who has a R7000 with Vortex firmware yesterday. SIP Passthrough was left at defaults, and he used version 1.30 of my guide. FPL is working perfectly fine for him with all FPL SIP servers. However, he did reset his router after flashing to Vortex.
FPL does have a relatively known issue for some people's accounts if they don't request a forced registration.
I use Asus Merlin. SIP Passthrough is Enabled + NAT helper (default) in my router as well.
Asus routers/firmware don't have a problem when SIP Passthrough is left at default settings.