If the SG300 does the routing between VLANs, the SG300 needs to be the default gateway of the devices.
On the SG300, use a seperate port & subnet, to link to the ER.
The SG300 can do DHCP, or the SG300 can do DHCP relay, using DHCP on the ER.
Better no re-use the same VLAN number for seperate networks.