Hi,
4G+ (or LTE-A, LTE Advanced) uses carrier aggregation (CA). So if CA is active, it means the router is using 4G+, if not, then it's using 4G.
You can check with this command: gsmctl -A AT+QCAINFO
When active, the response will look something like this:
+QCAINFO: "pcc",1348,100,"LTE BAND 3",1,128,-75,-9,-44,21
+QCAINFO: "scc",6300,50,"LTE BAND 20",1,127,-74,-8,-56,18
When not, it's like this:
"OK"