The Rail Planner app relies on companies sending their data to the European database. Since there is the yearly timetable change on 15th December booking windows are shorter.

You can check timetables on the companies websites. Most should be available there by now, at worst in 1-2 weeks.

You can book Eurostar/TGV reservations elsewhere than through interrail.eu -> for example Rail Europe (desktop) does it without fees.

Thank you so much that’s really helpful! I was going to buy a global pass for the trip.  Do I need to do reservations through the app if I have the global pass though?

The rail planner app can't book any reservations. It can only forward to some of the websites where you can book, including the Interrail website. However, booking reservations there has a number of disadvantages, such as it being more expensive than elsewhere.

The best overview for pass reservations is here:

https://www.seat61.com/interrail-and-eurail-reservations.htm