Is it possible to use Synergy KVM over LTE / hotspot networks?

If those mini-PCs have LAN ports you can just purchase a cheap 5-port switch.

Then you can configure the miniPCs with static IPs in the same range and plug them to the switch. The only thing needed is the IP and subnet mask. Note that the IP addresses must be on a different range from the Wi-Fi adapters; so if Wi-Fi addresses are for example 192.168.1.x, configure LAN adapters with 192.168.2.x 255.255.255.0.

As long as you don't change the default gateways (and don't configure new ones to the wired interfaces) they will communicate to the internet using the hotspots, and with each other over the LAN port. If they don't have LAN ports but have USB ports, you can get USB-to-ethernet adapters and do the same. USB-Eth adapters may require installing drivers.

If you don't want to add cabling, the same could be done by adding a second Wi-Fi adapter. The principle is still the same, but in practice it gets a bit more complicated.