+Log in to Windows AD domain from MG90

We use MG 90s in our ambulances. Each one supports a subnet specific to the truck an dis connected by an IPsec VPN back to our data center. When someone takes a laptop that they’ve never logged into before in a truck and tries to log into our AD the laptop says it can’t locate a domain controller. If they take that laptop, place it on our internal wireless network and login it works. They can then go back to the truck, get on its’ wireless network and login fine with the cached credentials. Is there some special config tot he MG90 that will allow a AD login if t he user doesn’t have cached credentials. We’re running 4.2.2/4.2.3 firmware.