- Make sure there is only one DHCP server on the network so that there isn't any contention. The home / SOHO router will most likely come with it enabled. You can use it, or the existing server. You can have 2 DHCP servers, with one, troubleshooting can be easier. It is not an absolute requirement.
- Make sure the router's LAN IP address falls within the range of the network. That way you'll be able to manage it from an existing station.
- Make sure your DHCP pool of addresses isn't exhausted. You can add more devices, but it won't help if there aren't enough IP addresses to go around.
You could also consider connecting the wireless router to the Net and plug the switch into that. That way all the devices could have access to the default 192.168.1.0/24 block of addresses and NAT to get out to the Internet through the router. There should be more than enough IP addresses to spare in that case.