There's no link between the seat reservations and the Eurail account unless you bought the reservations through your Eurail account. Where did you buy the reservations? 

I bought them on the train websites but because none of the timetables that I needed were synced with the app! This is really annoying! Thanks for your response

Eurostar doesn’t sell seat reservations themselves so in that case, I guess that you have bought ordinary tickets. How much did you pay? For which route?

No, I just have seat reservations, €31, for Eurostar. I’ve had a response from Eurail too to add everything manually!!!! So I’ll try that and see what happens. Thanks for your help

As ​@AnnaB says, there's no link between the seat reservations and your Eurail account/ trip planner. This is normal.

Add your journey in the Trip Planner. Your reservations won't show on Eurail though.

The Railplanner app is separate from your Eurail account. No reservations will show in the Railplanner app. In the Railplanner app you need to enter all trains, both with and without reservations, as journeys. 

You didn't mention travel date and time but if it's 15/12 or later, then just wait until the app has the new timetables. You only need to add trains in the app before boarding.