It won't be your Internet connection - the communications between your cameras and nvr are all local, so they never leave your house (in theory)
It's not even necessarily your router. WiFi is notoriously spotty and a speciality in itself. Some houses a setup might work perfectly.
Stick the same house, with the same WiFi setup but different microwaves, wireless landline phones etc into a different area and it just might not work.
A more powerful WiFi access point isn't necessarily the answer even if it is a signal strength issue.
You could stick in a mega powerful router that can shout its signal to 5 miles away. But the devices trying to send their data back can only whisper theirs for about 50ft, and not through walls.
Start at the beginning. Bring all your cameras into the same room as your router. If they work there, then the problem is signal.
Depending on their install location and how close they are to each other, you could install some more WiFi access points that can communicate with them more reliably and link them back to your router with an ethernet to power line adapter. A decent one should have more than enough bandwidth to cope with 6 cameras and you can use the same adapter at the router to bring in signals from several remote adapters, with an additional WiFi access point connected to each one to increase the WiFi signal in the area of the cameras.