I don't know why people aren't responding, is this such a silent forum?

Typically that setup that you describe should work perfectly.

In general any access point that can do "infrastructure client" mode should be able to provide that feature you give here, provided it supports also that 3G/4G modem. Moreoever, any access point that supports "WISP client" mode should be able to do the same while also providing a shielded-off LAN for you. The 3040 is said to support "WISP client" so it should allow you to create a LAN with it, this LAN is routed to the 3G/4G modem, and your pc has access to the internet through that 3G/4G because it is part of that LAN and its traffic is routed through the 3G/4G by the 3040.

That is how it should work. Please tell me if I'm wrong in any way.

PCs draw a huge amount of traffic though from the internet. Even just web browsing can net you about 100MB per day easily. Maybe 300MB per day is a better number. A modest amount of day-long web browsing will simply cost about 100MB. So make sure you have the data available. Also, make sure if you don't have unlimited data, that Windows has its automatic updates turned OFF. I'm not sure if this is even possible in Windows 10. It once cost me a big amount of money when my laptop started downloading stuff without me knowing it.

All your explanation is true and I've tried these suggestions. Everything has been running smoothly