The E298 error means there is no power to the Nest thermostat.
What type of heating system do you have?
With only 3 wires the Google Nest normally requires that a C wire be installed by using a spare wire between the thermostat and heat source or you can use the Nest Power Connector.
Some heat-only systems are only looking for a switch closure to turn it on. If that is the case, you will need an external 24VAC transformer and a small relay.
Thanks for your answer and for the helpful link, Patrick_Caezza.
egallacher, I'm chiming in to ensure that everything is covered here. Have you check on the link provided above? How's your thermostat now?
I haven't heard from you in a while so I'll be locking this thread if there is no update within the day. If you have any new issues, updates or just a discussion topic, feel free to start a new thread in the community.