R310 standalone doesn't provide IP address on guest network
We have a single R310 AP running standalone. First Wifi network is our company network, running on default VLAN0001. This works fine out of the box. Second Wifi network is a guest network (VLAN0020) I configured. The R310 is connected to a switch (port 23). It's port is configured to allow VLAN0001 and VLAN0020. It's also configured as a 'trunk' port. There is a second port in VLAN0020 on the switch (port 24), which is connected to our firewall. The firewall provides IP-addresses for clients on the VLAN, because it acts as a DHCP server.
My problem is that clients on the guest wifi network don't get an IP-address from the DHCP-server and therefore can't connect.
When I configure a third port in VLAN0020 on the switch and connect a client to it directly (Windows 10 laptop), it does get an IP-address, so the DHCP server is supplying addresses on the VLAN0020.
What do I need to configure for the R310 to provide the guest network clients with an IP-address from the DHCP server?
Thanks in advance!