@insmod Try to use computer. Connect to first switch and change swtich port to vlan 200. Next configure pppoe on your computer. Install wireshark and watch packets.
you have mishmash with vlan
correct configuration
modem0 vlan 100 access mode
modem1 vlan 200 access mode
igb0 switch port connect hybrid or general mode non tagged 100, tagged 200
interface on pfsense igb0 non tagged (for 100 vlan), igb0 tagged 200 (for vlan 200)
or
igb0 switch port connect trunk mode tagged 100,200
interface on pfsense igb0 tagged 100, igb0 tagged 200