VoIPo GS ATA confusion on IP address

I just installed a replacement home WiFi router here. The VoIPo service (Grandstream ATA) works fine, incoming/outgoing tests OK.
But
The ATA is dowstream of this router, as it was before. The LAN port is unused. The ATA's WAN IP is x.x.x.51, which is in the subnet created by my old and now by my new router.
The ATA says it is in DHCP mode. But power cycling it, etc, does not yield a DHCP request to the router. The ATA continues to use .51 without a lease grant. The old and new routers' DHCP server range begins with .100. I may have assigned .51 as a static IP long ago. But the voice prompt and the LAN side web server say that the ATA is in the DHCP mode on the WAN side.

Irrelevant, but the ATA's LAN side is x.x.3.x, different than the WAN side subnet.

Confused, I am.
PS New router is great: Cradlepoint 9850. I've used Cradlepoint travel routers where I work and their firmware functionality is really comprehensive - many times over what the mass market ones have. And I can stick my Verizon USB modem into this Cradlepoint if my cable modem service poops out.