I would ask if any SIM slot adapter will work flawless for the system as for 4G modems.
Let's say i will buy a well supported modem and i will them put on mpcie, will them communicate?
I have an WRT1900AC and it should have 2 mPCIe ports for wifi so i could put it inside an waterproof box. Or also, when are shown Rasperberries as supported, how they could give a SIM support if not present?
A lot of modems use USB despite the mPCIe connector.
You might want to open the 1900 up first and validate this claim. There were threads in the forum that not all of them have Wifi on m.2 slots.
As well as the USB issue, you would need a modem with a SIM card socket on the modem, or eSIM functionality. I think there are M2 to PCIe adapter boards that also mount SIM cards so the M2 card has access to the SIM.
The USB could be done by adding wires to the main board to take over the back panel USB and connect it to one of the PCIe slots instead. That would require more than beginner hardware modification skills.
All LTE modem cards use USB internally. So why opening up a device for no reason when you can use the available USB port to easily add a new modem.
For the records, only Linksys WRT1900AC v1 has mPCIe, and you would need to cut the heatsink. It’s so much work, and not worth the hassle.
If you don’t find a suitable USB modem, I could suggest some affordable external Huawei LTE modems instead. Something like second hand Huawei CPE E5186, and buy it some external antennas. It’s a Cat6 LTE modem and can support speeds up to 300Mbps. You can enable bridge mode, as well. You can find them on EBay for less than $50.
Other Cat6 models could be an alternative. They can easily fit a waterproof encasing.
Or the ZTE286D, delivering double the speed, for roughly the same price.
Assuming you can find one, wherever you are.
It can run Openwrt, too.
Ok i get it that is a really unecessary work.
But can you please, if you can tell me more about this
Becuse like i said in a previous thread i would like to buy this board: YY3568
I know is not officially supported, but i think it is. The point here is i want know if an previously attached sim slot, from production, can be better supported by the system, or eventually also just an adapter can do the work, because like @deeddy said, all LTE modem use USB internally, so, the only thing to see in a board is to have enough mPCIe slots.
No, the only thing to see is if the mPCIe slot is wired for USB.
Wow! Awesome! Cat12 device for that price is very cheap!
Can you run it in a bridge mode and get public IP, like Huawei? Based on this text, I assume it can:
https://tunkkaamo.cloud/index.php/2020/10/28/how-to-change-zte-4g-modem-from-route-mode-to-bridge-mode-and-get-a-public-ipv4-address-on-your-firewall/
Edit:
I just figured out it can run OpenWRT! Crazy! 
Thank you, @frollic !
I wrote it when I posted about it, but if you want bridge mode, stock fw might be a better option.
You can still "upgrade" it to "Nordic MF286D" fw, which seems to be the most recent.
The only issue is using it as bridged, probably user lose the band locks, i think APN will remain but i'm not sure about the only 4G lock.
Regarding the 1900AC i see that is supported by r00ter, and even if i build it with another board and use the 286D modem, can i connect r00ter and openWRT?
Maybe using it as bridged, i don't think this will lose any type of settings.