The DM200 has a built-in router, so you have two options for your setup.
DM200 as modem and router
The router is, by default, enabled on the DM200. You can use your DGND3700 as an Access Point. If you have a DGND3700v2, there should be an AP Mode checkbox in Advanced Wireless settings. Turn that on.
If you have a DGND3700 without the v2, then it may not have the AP Mode setting. You can still use it as an Access Point; it requires a few extra steps. Under LAN Setup, configure an IP address in the same IP subnet as the DM200. Make sure the IP address is outside of the DHCP range managed by the DM200. Otherwise, you risk the possibility of a duplicate IP address. Disable the DHCP server on the DGND3700. Make sure to click Apply to save each setting. Finally, connect a DGND3700 LAN port to the DM200. Leave the DGND3700 WAN port disconnected.
DM200 as modem only
Here, you will disable the DM200's router and use the DGND3700 as a router. On the DM200, go to ADVANCED > Advanced Setup > Device Mode and select Modem (Modem only). Then connect the DGND3700's WAN port to the DM200.
