We use MikroTik for this. All manner of interfaces including LTE and 5G are available. I hear you can connect USB serial to them directly, but we also drop a surplus Dell OptiPlex at each location and attach the serial ports to that device. Total cost is <200 USD per site since we already have the older desktops laying around.
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Hunter Fuller (they)
Lead Router Jockey
VBH M-1C
+1 256 824 5331
Office of Information Technology
The University of Alabama in Huntsville
Network Engineering