Technical question on external 3G/4G antennas for R7100LG 4G Nighthawk router

On advise of another user I am placing this post here

Hallo,

I am using this 4G router in my mobile home with which we tour throught Europe. But the problem is signal-loss because a mobile home is in principle a cage of Faraday (don't know whether that's the correct translation). So I would like to help the router with external antennas. As far as I can find all 3 antenna's on this router are used for the mobile 3G / 4G connection. 

My questions:

1. Is this a correct assumption that all 3 attached antennas are for 3G/4G connection? 

2. When I would buy external antennas's should I replace all 3 or is 1 or 2 also Ok? If yes, does it matter which one?

3. Do you have any advise on which I should purchase (I think it needs to have a RP-SMA Male connector)?

4. Are these antennas always for only 3G or 4G or can this also be combined?

Hope to receive some advise. Thx.

Best regards,

Erik