Help needed with DHCP relay with VLANs

Hi guys, I've already done tons of searching in many forums but I couldn't managed to get this thing to work 100% I am probably missing something.

Here is the scenario: I want to broadcast 2 SSIDs each one in different subnets, Native VLAN is still using ID 1 (192.168.160.0/24) and a second VLAN with ID 10 (192.168.20.0/24), and this is what I got:

1x Dell PowerConnect 6248 

1x Firewall doing the routing (plugged to port 1/g29)

1x Unifi UAP With 2 SSIDs (plugged to port 1/g10)

1x Windows Server 2012 R2 DHCP Server (192.168.160.151)

What I did:

1. Created a VLAN with ID 10 in the PowerConnect and assigned an IP in the 192.168.20.0 subnet
2. Each VLAN has an IP address native VLAN is 192.168.160.249 and VLAN 10 is 192.168.20.249
3. Configured to General the port where the firewall is plugged to allow untagged and tagged and left the PVID to 1
4. Created the VLAN 10 in the firewall and assigned an IP int the 192.168.20.0 subnet to it
5. Configured one random port within the VLAN 10 to test the routing, connected a laptop to that port and configured a static IP address in the 192.168.20.0 subnet and I was able to reach the other subnet, so the routing is OK.
6. Configured another scope in the DHCP Server to serve IPs in the 192.168.20.0 subnet (tried with and without superscope) 
7. Configured the BOOTP/DHCP Relay Agent in the PowerConnect and set the laptop to dynamic IP, it was able to get an IP in both subnets (wired)
8. Configured one port to General with tagged vlan 10 and PVID 1 and plugged the Unifi on it
9. Configured two SSIDs on the Unifi, one without VLAN ID and another with VLAN ID 10
10. Tried to connect to both SSIDs, sometimes I was able to get an IP, sometimes don't, and that's is where I am stuck, I guess that if I configured everything correct I shouldn't have this issue, where sometimes work and sometimes don't.

If anyone could help me I will be very thankful.