It would be best for you to catch one of the DIRECT trains from the FRA Regionalbahnhof station. These are RE (regional express) trains which make the trip in 90 minutes - 11:24 > 12:54, for example. You probably will make this train if your flight arrives on time. Of course, that's anyone's guess. But the good thing about using regional trains only is that ANY regional train ticket is valid at any hour of the day. If your plane is late and you miss the 11:24, then you just catch any regional-only option after that. 11:30, 11:45, and 12:30 are the regional departures offered on the date I just checked. Those however will require a change of train, either in Mainz or Wiesbaden. The next DIRECT regional express option after that is 13:24.
Do not buy two one-way tickets at the normal price (which will cost €70+.) What you need to buy is the Day Ticket for Germany (€59/2) which is actually a day pass valid for ANY regional trains you wish to use that day.
https://int.bahn.de/en/offers/regional/day-ticket-germany
Also, do not buy this day pass in advance. Buy it once you are on the ground at FRA, either from a ticket machine at the station there or using the DB app. The day pass is non-refundable. If your plans change for any reason and you cannot travel that day, you lose the ticket. So just be patient and buy it there. You get no seat reservation on regional trains, no matter when you buy the ticket, and no matter whether you buy the day pass or the normal-price one-way tickets, so there is ZERO advantage for pre-purchasing.
