SNMP is notoriously sensitive to timing. If your WAN link has 800ms of latency and your timeout is set to 500ms, you will get Error 2003. Similarly, 1-2% packet loss can kill SNMP requests.
typically manifests as:
Many devices have "Allowed Hosts" or "Management Access" lists that explicitly define which IP addresses can query them. If the PRTG probe’s IP is not on this list, the device will ignore the request. paessler snmp error 2003