Cannot RDP from non-domain to domain computer through RED 15

Question

SG230 UTM 9.510-5 
 I have currently have 3 RED 15s deployed with standard-split tunnels that are all functioning properly. They are used for server file access and RDP. 
 I recently deployed a 4th RED 15 with absolutely identical configs to the other three. The only one difference is that the remote computer (behind RED) is not a domain computer. The user can access the server files and map a drive but cannot connect to their domain desktop via RDP. 
 Using nslookup from the remote (non-domain) computer, the domain computer is identified with it's name/IP, but pinging the domain computer name and IP fail. RDP fails with "cannot find computer on this network...". Sophos software VPN client works perfectly (but dreadfully slow). 
 I tried both the DNS name and IP address in RDP but get the same result. 
 Is this related to non-domain->domain authentication or could there be an underlying DNS issue I am missing? 
 Thanks!

TomMorgan · Accepted Answer

This issue was not resolved with my last post since, unknown to me, the employee was still using the Sophos software VPN after I deployed the new RED. Upon discovery of that, I found that the new RED was still unable to make the RDP connection. 
 With the help of Sophos support, it was found that the remote computer was not able to respond back to the RED network. The solution?? Add a route to the remote computer: 
 route -p ADD [RED subnet] MASK [RED netmask] [remote network gateway] 
 route -p ADD 192.168.10.0 MASK 255.255.255.0 192.168.50.1 
 BOOM! Instant connection to RDP.