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there is no need to make the connection in the SLM into a LAG, unless that is what you need. if you do create the LAG the 1800 router needs to also understand that it is connecting to a LAG (ether-channel) using 802.3ad, but again this is not a requirement for inter-vlan routing.
make sure that all your subnets (vlans) have the default gateway set to the sub-interface of the 1800 and that the 1800 has a default route configured (may already be configured).
so should be something like this:
ROUTER:
fa1/0.1: VLAN1 -- IP: 172.16.10.1/24 encap .1q
fa1/0.2: VLAN1 -- IP: 172.16.20.1/24 encap .1q
etc...
SWITCH:
g24: TRUNK allow VLAN 1,2,3 etc...
remember to tag all VLANs (except vlan 1 ofcourse)
Your clients will have the gateway IP of the subinterface in the router, such as 172.16.20.1 for example.
see if this is what you have and let us know.