SmartSDR server in a 4G network

It will all depend on the service provider and quality of 4G modem. The issue with Wireless is the network reliability from a packet loss, latency, and jitter will be variable. At the very least, plan on a 4G directional antenna pointing at a known tower to mitigate signal bouncing around/line of sight issues.

The biggest challenge you're likely to encounter is not so much the network performance, although as others have noted that is a possibility. I use such a configuration at home and the ISP which is in my case reselling AT&T wireless services, provides me with a "natted" connection i.e. my IP address is not directly accessible as a public IP on the Internet. It is instead a "private" IP address located behind their "router" somewhere in the cloud. SmartLink requires port forwarding be configured on your router, but since their router is also in line somewhere out there in the cloud, it would need to be configured too. They will provide a true "public" IP address for an additional monthly charge. This was the same situation a few years ago when I was using Verizon wireless as well.