Maybe … try to delete the first port forwarding rule, reboot the router and despite everything, also activate the UDP protocol, which apparently seems to help sometimes:
Port 5000 is used by the Plug and Play service itself so it probably not available.
Try 54999. I use something in the 49000 to 50000 range after verifying nothing else in the network is using it.
I get this on iOS and Android if anything changes in my server or network setup doesn’t matter what you do it won’t connect. I think the app caches some things and doesn’t update them properly. Deleting the app or all its data tends to get it to work again I find.
But - if nothing helps and I really want to get the thing working - I would definitely reset this ■■■■ router and all other network components, disconnect everything from power and then reinstall ARC.
2 port scans shows the port as open, with a third timing out - not sure what this means?
I’ve reset my router, disconnected my rock from the power and deleted arc. Then I disabled upnp, enabled tcp & udp, set up a manual forwarding rule, switched the rock back on, entered the relevant port no in the ARC setting in the Roon app, downloaded ARC and logged in and connected to the rock while on my home network. I then tried to connect using 4g and still the same issue - it can’t connect.
I found the information that PortScan is a security tool, with the aim to find open ports and eliminate security risks.
Not a tool that proves accessibility from the outside.
Port Checker is also intended to detect open ports in order to test if port forwarding settings on a router are working.
In contrast, the application whatsmyip serves the purpose of proving accessibility from outside. Which is obviously not the case here.
You may not be getting a real public IP address from your ISP, but rather you are behind a CG-NAT?
You could contact your ISP to clarify what’s going on.
Carrier-grade NAT (CG-NAT) is a network address translation and port forwarding method that allows several users to share one public IPv4 address and this usually blocks access from outside.
I’ll give them a call in the next few days and hopefully find out what’s going on - thank you for your help 
All the best and I hope everything will work soon!
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