Actually the error message you found is very helpful. It means that the plugin cannot contact your wallet.
Reasons for this can be:
rpcallowip= line in your conf. You probably have something like
rpcallowip=188.8.131.52/24 in your conf. But if you’ve set up NAT then connections should be coming from your modem/router. Try adding something like
rpcallowip=192.168.0.1/8, assuming this is the IP of your router. Whatever you do make sure to use CIDR notation (i.e. include a netmask as shown above)
2. NAT configuration. Double check the port forwarding rules in your router. You want port 8332 to be forwarded to your local network address.
Hope this helps.