There’s almost always seats available somewhere, even if you have to go and have a coffee in the restaurant car. But it’s certainly worthwhile considering for peace of mind.
On the DB website, run your query but select ‘book seat only’. You should see a bunch of results priced at €4.90, including your 0955 departure.
When I search, it suggests you change at Cologne/ Köln - which is a better idea than Frankfurt Airport anyway.
(Ignore anything that involves a leg on TGV or EST.)
Buy that, for €4.90, and you have your seat reservation.
You also need to add the trains to My Trip (assuming you have a mobile pass?) - that’s what generates the QR code which is your ticket. (The seat reservations on their own aren’t good for travel.)