Can you still return the watch? Else you may need to buy another used phone. You can also compare what features are supported, maybe TomTom can tell you why it’s not working or if it’s just a platform issue.
I tested a few BT features with a modified Android 4.4.4 and it was pretty slow and disappointing. Maybe a software update will work, but it was promised [ages ago.] (https://www.fairphone.com/2015/09/16/long-term-software-support-fairphone-1/) The last time I heard about it was end of January 2016, but I assume they will first focus on getting the FP2 up and running.
I think the FP1 will need updated BT firmware from MTK … I’m not sure if this will be possible or if this firmware even exists, I’m not a BT expert. Funny thing was that it worked a tiny bit … just very, very, very slowly. Connecting and everything else work very randomly and very slowly (I tested a thermostat, not a watch, by the way).