Hi @Hans_Ericsson,
Have you tried using the Bullet M2HP standalone? UniFi/UI/Ubiquiti got out of producing the Bullet series in the traditional form a few years ago, and the ones that still exist aren’t usable for anything other than point-to-point connections to other Ubiquiti products. Perhaps that is what you’re using it for? If you aren’t and intended on grabbing remote WiFi signals, it’s possible you have one of the newer units that won’t do that…
A Bullet or similar device should work if connected to the Nighthawk LAN port and with offloading enabled. Even though the Bullet might be doing normal router functions, it’s only client is the Nighthawk, and not your end clients directly.
There’s also an option to turn things like the Bullet into “bridge” mode where they act as a client, and not a full fledged router, which I’ve done with devices connected into the Nighthawk, but it does take a bit more networking skill.
I personally would go the route you brought up - a central router for your local network, and have the Bullet and Nighthawk plugged into that router as WAN sources. That way you can upgrade your Nighthawk at some point down the road, or add a second one, and not have to reconfigure or mess with things.