Using a Netgear 4G modem with Netgear D6400 router

> [...] The existing router has an onboard modem and it's not clear
> how/if this would need to be disabled if the unit was working with the
> 4G modem. [...]

   A better way to look at the question might be to ask how the router
decides between the DSL modem and the Ethernet WAN/Internet port.  In
the D6400 User Manual, there's a section, "Change the Physical WAN
Connection Preference" which says:

      The modem router can autodetect the type of WAN connection.
      However, you can also set the WAN connection preference manually
      to either the DSL port or the Ethernet WAN port.

   I don't know what "autodetect" actually means, so I would not bet
that the router would automatically switch between the two WAN
connections when the (fiber) DSL link dies and re-awakens.  I haven't
tried it on my D7000, so I know nothing, but my guess would be that an
Ethernet connection would be preferred over a DSL connection.  (But what
do I know?)

   I suspect that you could use the D6400 either way, but you might need
to switch it manually between the two WAN interface choices (ADVANCED >
Setup > WAN Setup : WAN Preference).  I haven't looked, but I'd also
guess that there are more sophisticated (industrial-grade) routers which
can be configured to do what you want (automatically).