I had the same problem: Rachio 3 could not connect successfully with a FIOS Router. I told tech support I was running through a WiFi repeater (mine is a Netgear Nighthawk), but no one told me the solution above which was to connect to FIOS router directly. I tried everything: making rachio DMZ, lowering firewall security, setting up a guest network, etc… and nothing worked. 8-10 hours of troubleshooting including ultimately re-wiring my Fios access point to another area of the house closer to my Rachio and it finally worked. I’m now looking forward to see what the Rachio can do.
Franz and Rachio tech support: make this a high priority bulletin for all new customers. I was so frustrated with the Rachio that I almost had my sprinkler system installer take it back and give me a regular (non WiFi) controller. Seems Rachio needs to be in the home network business as much as the Smart sprinkler controller business. Get your helpdesk people to know the ins and outs of Fios and Comcast home routers intimately and you will save customers a lot of frustration.