Tips for the OP:

1. There are always a small number of seats which are unreserved - you may get lucky and grab one of these. On this train, these tend to be seats 46 and 55 (amongst others) in coach 28.

2. Some seats are reserved for "BahnComfort" (DB loyalty card) holders - you may sit in one of these and will only have to give it up if a BC cardholder shows up. These tend to be seats 54 and 56 in coach 28 (and maybe some others).

3. You can sit in seats marked as reserved for disabled passengers unless they are needed.

4. You can sit in seats indicated as reserved for last-minute reservations, unless someone shows up with a reservation for that seat. (Note that quite often this happens).

5. After leaving Frankfurt, look out for seats which are reserved from Frankfurt but not occupied. After about 15 minutes, the reservation will disappear from the electronic display, and these seats are then free for anyone to occupy.

Check the seat plan for seat locations.

I travel on this route regularly, generally with an Interrail pass, and I never reserve a seat. I usually make a beeline for the "BahnComfort" seats (seat 56 being my favourite) and have only ever had to move once (to the seat opposite).