Can you post exactly what your doing and why you think it' s a problem with the FGT?
Is the a problem with the
LAG or
DHCP related ( config a static host when your LAG enabled, can you ping your FGT inside interface once you allowed icmp {172.20.10.100} ? ) ?
Is the DHCP server bond to the correct NAMED interface?
( we or at least I' m not 100% sure on what your problem is based on your 1st paragraph details )
But once you build the virtual interface, you can craft sub-interface specifying the virtual-interface as the parent and use that for your vpn,dhcp,etc........Just like with any other interface(s)
Since you said netgear are you 100% sure it supports 802.3ad link-aggregation?
Is the netgear active or passive in support of this?
Also are you static or dynamic LAG groups? ( the former is not 802.3ad ) ?
( iirc it like a cisco-switch, does a static
no-protocol LAG assembly .... that will not work with a fortigate )
FWIW
iirc on a limited number of SOHO device did not support LAG interfaces. I would guess a FGT100D is not one of them.
Also, I never seen problems w/fortinet support for 802.3ad btw.