It's possible your modem is full (see this report) but mmcli commands won't do anything about it. You'll need to use minicom or picocom to send AT+CPMS? to the modem at /dev/ttyUSB2. A response like
+CPMS: "ME",2,99,"MT",2,99,"MT",2,99
means there are 2 messages out of a maximum of 99 in the modem. If the modem is full, you can purge it with AT+CMGD=,2
If it's not full then the problem lies elsewhere.
+CPMS: "ME",2,99,"MT",2,99,"MT",2,99
means there are 2 messages out of a maximum of 99 in the modem. If the modem is full, you can purge it with AT+CMGD=,2
If it's not full then the problem lies elsewhere.