It is advised to scan the device by directly connecting to the WAN to avoid unpredictable behavior when doing a scan from/through the Internet. There might be a port open on a router in front of the Peplink SOHO or some ISP may intercept the traffics at different times and you may see the port is opened. Directly on the WAN port means taking a laptop or pc, and connect a cable directly to the router, and do a port scan from your system LAN port to the router’s WAN port.
Examples of similar cases: