@AndrewZ
Thanks. I did not even think about an internal card and right now think this is my last option.
I am wondering whether to use a USB modem like http://consumer.huawei.com/en/mobile-broadband/dongles/features/e3372.htm or a router like https://www.amazon.com/Huawei-B315s-22-Unlocked-Mobile-Router/dp/B01B5OM94O/ - whether there are any implications.
@BlueKobold
Thanks, buying a new CPU board is a last option after buying an internal card. ;) To me, there seem to be several drawbacks, like kind of a high price, and also due to high integration I would have to replace the whole thing if any one component fails, either on the mainboard or with the LTE ports.
Also, since our current firewall has just now reached it's limit at the same time that I have started to look into pfSense, it will be a Hyper-V based solution and I will just put some more NICs into an existing server, just the LTE part is not solved. (Using existing hardware makes process of introduction faster, also, since this will just be a VM and some NICs, if anything fails, I can either move the VM to another host and/or replace or move some NICs.)