From Paris CDG airport you have to drive with a train not included in Eurail to Paris Est. Which trains do you want to get? Maybe we can give some tips where to book. 

You can use ANY source that will also de just REServ-these Qs pop up here more as once daily and there are 100s of posts giving details. It is NOT a matter of via seller or when there-in fact the last is highly unwise as trains may be fully booked.

You should note that on SNCF TGV=superfast trains there are 2 types of R$EServ: 10€ for IN the quota (small), and 20€ for beyond-as long as there are seats left. The app canNOT handle the latter and then tells that its non available.

there are also a few TGV from Airport station to the east of Fr (Strasbourg or maybe Mulhouse)-that can bring you on the way without that nasty change in Paris.

DO be advised that flites from over the ocean may arrive very early-or more often very, very late, this makes making a fixed REServ a nasty thing. Plus that CDG is an utter confusing airport with many terminals and the train station is not linked to most.

After some fumbling around I did manage to figure out how to book our reservations. 

The train we will be boarding is TGV 5482 it runs from CDG to Strasbourg.

It might be pushing it as far as landing and getting through customs and then making the train. But if we don't make it we will figure it out. We have 90 minutes from plane landing to train leaving