We have a couple of choices on trains from Prague to Dresden, the question is the time required to change trains at the Dresden Hauptbahnhof. We could take a train that arrives at 18:45 hr or another than arrives at 20.47 hr. We have to board a CNL from Dresden to Frankfurt at 21.04 hr. Can we get from one train to the other and get on board in 17 minutes. Alternatively we could take the train from Prague that arrives earlier, store our luggage at the Dresden station and go into town for a couple of hours. What do you train aficionados think? We can't miss the 21:04 train.
You also asked this question on "To the West". To avoid confusion, you should delete it there. According to the Bahn website, that night train, CNL 458, starts in Prague at 18:29. Why not start with that train in Prag? If fact, that night train uses track 17 in Dresden, the same one as two "trains" from Prague, D 60456 and IC 61458, which arrive at 20:47 on track 17. I'm pretty sure, looking at the schedules for all three "trains", that the two trains arriving at 20:47 are actually separate sections of the CNL train. So, if you take the IC, you will actually be on the same train. But why not just take the CNL train from Prague to begin with? Your problem is probably that you are using RailEurope as a schedule site. Don't!!! You can book the whole trip (the CNL train from Prague to Frankfurt) online on the German Rail (Bahn) website. In case you failed to take the advice on this board, and are trying to do this with a rail pass (reservation only), you cannot book accommodations on the Bahn site. However, if you just get on that train in Prague, it will be the CNL train. The conductor, upon seeing your accommodation reservations for Dresden to Frankfurt, should let you move into your compartment before Dresden. Note, because the train leaves Prague at 6:29, before 7 PM, you will have to use two days of your rail pass.
I assume you are using a rail pass. If not, just book the whole trip, Prague to Frankfurt, online with the German Rail (Bahn) website as a Sparpreis fare (94€)/person in an economy double). If you are using a rail pass, and you haven't already done so, you can book a compartment (rail pass reservation supplement) for Dresden to Frankfurt online from the German Rail website. This will cost about 75€ (~$100) per person in an economy double compartment. Put in Bad Schandau (Germany) to Mannheim. When you get the connections, it will say "Check Availability" after the CNL train. On the next page, pick "Book only extra charge" to get to the page for rail pass reservations. In Prague, buy a ticket on that train for Prague to Bad Schandau. That should cost about 15€/person. You'll get there at about 8:15 PM, from which your rail pass is valid for the night train in Germany according to the "7 PM rule" (be sure to fill out your pass for the following day). Although you booked your compartment through to Mannheim (to satisfy the "no connections before 4 AM rule), don't go all the way there. Get off at 3:59 at Frankfurt Süd (south) and get on any train going to Frankfurt Hbf (could be an S-Bahn).
Thanks to all.
After some agitating time with my Credit Card vendor, I did get the problem solved. I needed to get a deluxe 2-bed compartment booked, and finally did so by booking the reservation for the "sleeper" separately (from Dresden to Frankfurt) as the same train was fully booked from Prague to Frankfurt (via Dresden). I don't understand the nuances of booking from Prague vis-à-vis Dresden to Frankfurt, but it's done, so the objective was achieved.