Antenna for LTE modem

I've recently bought a strongish directional antenna for my LTE modem,

and the cables to connect to the modem.

But when I connect it to my Sierra Wireless 7455 card (which is working fine with small omnidirectional antennas, connected to my OpenWrt router) the connection drops and it does not come back.

When I connect the same thing to my Huawei B310s-22 LTE modem (which is a full device with its own web interface and ethernet and wifi capabilities, it's not running OpenWrt of course) then it works correctly.

Another weird thing is that while this antenna claims it's MU-MIMO whenever I connect both cables the signal intensity drops dramatically, so I suspect that this "dual antenna" is just supposed to be used one at a time, depending on how the antenna is oriented (horizontally or vertically, as that's what is labeled on the connectors).

I might just saw the metal plate and make two separate antennas out of this thing later if I figure out why it's not workign with the Sierra Wireless modem card.

I'm suspecting that this antenna requires some signal booster circuit that is present in the Huawei full device but is not present in the Sierra Wireless M.2 card.

Since I'm a goat in this field and Google is failing me, does anyone know what is the problem?

Some modem can measure the signal strength with an additinoal pcb parts on the antenna and on the modem. Maybe the original antenna has this and the modem due a check? A PLS8e from does have this feature.
https://www.gemalto.com/m2m/solutions/modules-terminals/industrial-plus/pls8

I doubt it, as the antennas it works with are the dumb, simple and cheap 2 dbi ones that look like the 2.4ghz wifi antennas.

The Sierra Wireless card I'm using is designed for laptops and other such devices (it's a Dell branded card that I reflashed to generic with https://github.com/danielewood/sierra-wireless-modems ).

How do you know the cables are working? If I found the correct specs, then the antenna comes with N female and the B310s-22 with SMA female connectors. The EM7455 has MHF4 female connectors of course. So you must be using different sets of cables. And MHF4 to N is somewhat uncommon, and most likely handmade by someone... So how do you know they work?

The sierra modem is in a usb 3.0 metal enclosure like the one depicted in the link above, that has SMA-MHF4 tiny cables inside, see the images.
The modem loses connection as soon as I detach the 2dbi SMA antennas so I think this part is working fine.

I have bought a couple N-SMA cables (50 cm) and I'm using the same cables for both the B310s-22 and the Sierra modem.
These:

which from the manufacturer specsheet https://www.interprojekt.it/rsc240-coaxial-cable-p-906.html
should be the right kind (50 ohms) for the antenna.

But maybe I've missed something obvious. As I said this isn't really my field.

Right, that makes sense. So you are actually using N to RP-SMA and RP-SMA to MHF4 cables for the EM7455, and both cables are tested. Then I have no clue what could be the problem.

So, just to confirm, I should be able to connect the Sierra Wireless 7455 card directly to any antenna rated for 50 ohm and it should work, right?

Because at this point I'm going to send back the current antenna as "not working as intended" and try with another one (a yagi maybe).

I am sure there are other restrictions than just impedance matching. There is a whole "Antenna Specification" appendix in the EM7455 Product Technical Specification. You have to match the gain of the two antennas for MIMO for example. And the two antennas should have more than 10dB isolation from each other. Etc. Etc.

It's probably also possible to overload the receiver, although I'm not certain you can do that with normal antenna gain. But this should be easy to test with an attenuator. Be aware that the EM7455 probably is designed for antenna systems with gain around 0dB (including cabling). Using a high gain antenna with short cables isn't necessarily a good thing....

so I suspect that this "dual antenna" is just supposed to be used one at a time, depending on how the antenna is oriented (horizontally or vertically, as that's what is labeled on the connectors).

no, that is for cross polarization which is, in your situation, probably a good thing. if you are using a directional antenna you need to aim it at your base station.

I know what "directional" means, I already pointed the antenna around in a full circle.

If both connectors are in use, I don't get any signal and no connection even on the Huawei modem-router.

I've already sent back the antenna as "defective" though, as it's not really working properly even with the Huawei modem-router, heck, it's getting better signal with its own internal (still directional) antennas.