Yes, you can connect any Open Mesh access point directly to a modem and it will handle DHCP to Open-Mesh repeaters and client devices. However, we recommend putting a router in place to use multiple gateways, take advantage of bridge mode, use port forwarding and firewall features, etc.
See "Should I use Open Mesh access points as a router?" for more information on limitations.
note: This topology relies on the ISP using DHCP on the backend. If this changes for any reason, the entire network could go down.