Seeing it on your PC
The WireGuard app or a VPN provider's app connecting out on 51820.
Should you block it?
Normal outgoing for a VPN app.
Check it on your PC
Run this in PowerShell to see what is using port 51820 right now, and which program owns it:
Get-NetUDPEndpoint -LocalPort 51820 -ErrorAction SilentlyContinue | Select-Object LocalAddress,LocalPort,@{n='Process';e={(Get-Process -Id $_.OwningProcess -ErrorAction SilentlyContinue).ProcessName}}No output means nothing is using the port at the moment. For every connection at once, paste netstat -ano into the netstat explainer.