Using the Almond Plus as a second router

[madmartian]

I have a main modem/router with DHCP turned OFF and several cameras with static IPs connected to it. My goal is to take my Almond Plus and connect to the network to allow for a few devices that cannot set a static IP to connect to. But how do I do this in a way that allows me to access those devices from the main network (a computer plugged into the main router, not the Almond Plus)? Currently devices connected to the Almond + can access the internet, but I cannot access the devices via a browser on a machine on the main network. I can't even access the Almond+ itself from the main network, even though it has a static IP on the main network.

[madmartian]

Here is possibly a better way of explaining my setup and what I am trying to accomplish:

The goal:
1. Primary modem/router: Arris SBG7600AC2. Setup with static IP with all connected devices setup with static IPs. I already have this setup.
2. Secondary router: Securifi Almond Plus. Setup with DHCP to assign IPs to devices that do not support static IP as well as guest devices (since I don't want to make guests setup their phones with static IP). I already have this setup.
3. Devices connected to the secondary router need to be reachable from a computer connected to the primary router. This is not yet working.

What do I need to do to the two routers to make step #3 work?