Rail Europe works very well, I've used it multiple times for passholder reservations. I'd book ASAP as TGVs to Brussels are really popular and have a limited passholder quota.

On Rail Planner you only "log" the train you'd like to use: it doesn't guarantee you a seat or anything. This can be done at any point prior to boarding.

Could be that this train is not yet bookable in the rail planne app due to the timetable change happening around 15 december.

Note that a reservation is quite pricey on this train (20 EUR), and that ordinary (non interrail) tickets are quite cheap now (30-35 EUR). If this is the only journey of the day with a pass (a pass day easily costs 50 Eur per pass day), you might consider buying an ordinary ticket and save your travel days for more expensive or better value routes.

There can be big gaps in the service CDG - Brussels, if you don’t want to wait too long, you can get to Lille and continue to Brussels by Belgian regional trains.

Thank you for your replies, I really appreciate it, I have my Eurail pass with an extra day, so it doesn’t bother me to use it for this.  Your advice is greatly appreciated