Don’t panic, this is still going as planned as this is totally expected and this is the reason why:
We are now in a situation where we have the Primary server running 2017 but one (for us) or possibly more than one for you running on 2016 , its not possible for the 2016 server to synchronize as its databases have not been upgraded yet, they will therefore be stuck in recovery but we are about to fix that very soon.
This is a viable upgrade option: we did it when upgrading from 2014 to 2016. There are a lot of steps, but in the end, it worked fine.