Usually, the easiest way is to configure the ISP's Router to put the AyrMesh Router into its "DMZ" or "Static NAT" area. This is normally a "two-step" operation. In the rare cases this proves difficult, you may choose to "bridge" the other device, or to "port forward" ports 80, 22, and 9001-9099 to the AyrMesh Router.
Below you can download a document called "General Residential Gateway Notes" that explains how these devices work and what the technical problem is, if you're interested. It also outlines what can be done in general terms.
Also below are specific guides for doing this with several popular "Residential Gateway" or "Combination Router" devices used a lot by ISPs. Download the one that corresponds to the equipment you have and it will guide you through the setup of that equipment.
Note that many companies use Westell equipment; some of them use the Westell interface (blue and white) and some use the Verizon interface (red and black). While they work approximately the same way, the interfaces look and behave a little differently, so there are two documents for the Westell routers.