It looks like IPsec, and the instruction seems to be here.
Unfortunately, I have no experience with IPsec.
Try to follow the @mikma's advice.
And make sure to open ports on the destination host if possible.
Checking UDP ports is typically more problematic than TCP and should be performed differently and separately from usual TCP scans.
UDP port check may not be supported by most online port check services.
Moreover, you need to open the destination, forward it on the router and run the software that listens on that port.
Something like that, though I'm not sure if 4500/UDP is required to open explicitly or not.
Perhaps it depends on the specific IPsec implementation.