What is not clear to me is the fixation with vlan20.
Is this a vlan the ISP provides internet traffic tagged with vlan20
OR
is this just the vlan you setup on the switch to move ISP traffic from the modem to the router.
Regardless, its not the switch that decides anything, the entire DHCP setup and vlan setup is done at the ROUTER and
a trunk port goes to the switch to
a. receive ISP traffic from the modem ( you used vlan20 )
b. to receive the management vlan from the router ( all smart devices should get their IP from the management vlan )
c. to receive any other data vlans the 5009 creates for dispersal on the rest of the switches ports.
Now with the switch behind the router, nothing much has changed except that vlan20 to carry the ISP traffic to the router is no longer required as there is a direct connection.
The management vlan and data vlans are still required on a trunk port to the switch.
http://forum.mikrotik.com/t/using-routeros-to-vlan-your-network/126489/1