Access RED LAN network from local LAN (RED WAN TO RED LAN)

I have a customer that uses SOPHOS FIREWALL  and have connected a bunch of RED 15 and RED 20 devices. 
And for my customers end users they usualy connect to their RED device using SOPHOS CONNECTS . 

But mor and more often , they ask if it is possible for them to connect from their own network to the Local RED's LAN network, and access the devices on the RED LAN nework.  AT minimum they want to acces a spesific service, but if it is possible to acces "all" devices on the RED LAN network it whould be great. 

My question are if that is something that is possible to achieve.  CAN Customer network acces the RED Network trough WAN ?
The setup is usualy something like this. 

SOPHOS FIREWALL --> Customer ISP --> Customer Network --> RED device --> RED LAN .