I also only have a GX20 from Cisco. WiFi is provided by a Ubquiti AmpliFi HD running in bridge mode.
The first time I set it up, I connected to it using.a cable from my PC, because I had to change the settings for the ISP which can be found on the Configure tab on http://wired.meraki.com. This was the set up:
--------- -------- ------
| Modem | <-> | GX20 | <-> | PC |
--------- -------- ------
Second, and subsequent times (because I had to reset the GX20), I found that just having the GX20 on the existing network, the Meraki Go app finds it just fine, although I still need a web browser to change some ISP settings and prepare it as the gateway device to the internet. First, like this:
--------- ----------- --------
| Modem | <-> | AmpliFi | <-> | GX20 |
--------- ----------- --------
And then like this once the GX20 is ready to act as the gateway:
--------- -------- -----------
| Modem | <-> | GX20 | <-> | AmpliFi |
--------- -------- -----------
The security subscription can be activated from https://account.meraki-go.com/.
HTH