Update 2018-04-06: Our modem became unreliable, and today an RCN tech came to swap it out. The new one doesn't have this issue. The tech said when they first started distributing these they had all sorts of reports due to weird factory settings, but that there's been a firmware update and the new ones they're distributing are much less trouble.

I saw that gigabit service is now available in our area with RCN. [1] Unfortunately, when they installed it, We couldn't ssh. It looked like RCN was blocking port 22.

After some looking I found this thread where it turns out the answer is:

1. Visit the url printed on the modem and log in with the supplied username and password. On my box this was 192.168.100.1, admin, and [redacted].

2. Open "Gateway > Advanced Settings > Firewall" and turn off Firewall protection for both IPv4 and IPv6:

   

3. Open "Gateway > LAN > DHCPv4" and change "LAN Firewall" to "Disabled":

   

[1] Unlike most of the country we have two cable providers to choose from, RCN and Comcast, which means that Comcast here is actually pretty good. But I still don't like Comcast, and won't use them if I have a choice.