Internet and MPLS configuration on SOphos XG 210

Good Evening,

I have a Sophos XG 210 firewall on my network. ISP has a single router both for Internet and MPLS as below.

1. There is only one cable from router to firewall carrying both MPLS and internet traffic.
2. All routing to branches done on the ISP router.
3. TO LAN cable from the router was connected directly to the switch before introduction of the firewall.

Below are configs on ISP router:

interface GigabitEthernet0/0.1104
description POT_INTERNET
encapsulation dot1Q 1104
ip address 41.150.107.105 255.255.255.0
ip nat outside
ip virtual-reassembly in
!
interface GigabitEthernet0/0.3663
description POT_MPLS
encapsulation dot1Q 3663
ip address 172.20.100.102 255.255.255.252
ip nat inside
ip virtual-reassembly in
!
interface GigabitEthernet0/1
description IMA_LAN
ip address 172.12.15.1 255.255.255.0
ip nat inside
ip virtual-reassembly in
duplex auto
speed auto

QUESTION:

I need to configure the firewall on port 3. How should i do it?(What IPs do I use?). Port 1 on the firewall goes to LAN.

Thank You in advance.