According to your description, you have two VLANs, one for each wireless network (guest and corp) and you have set up a DHCP server on the Sophos firewall to provide IP addresses. Here are the questions I have for you:
- Is each wireless LAN on a different subnet? If so you need to create a separate scope for each one.
- If there are two subnets, and you have created only a single interface on the firewall with one IP address, then you will require an IP helper address to help your clients find the single DHCP server from both of their subnets.
- in order to verify or test connectivity, can you use a laptop plugged in to the port on the switch where you have your access point and set up a static IP address in the correct subnet. Do you have connectivity with the default gateway?
- If your wireless clients are not getting IP addresses via DHCP, could it be that the access points are not configured in bridge mode so that clients will get their addresses from the firewall?
I hope these questions will help in determining the root of the problem and help you on your way in troubleshooting.
I hope this has been helpful!