So you have a Fendt from a few years back before they changed from Topcon hardware to Novatel/Trimble. The tractor is auto-steer capable with a VarioGuide Precision receiver (Omnistar HP supported and the gyro unit included). A part time farmer driving a Fendt, very much like myself but why do I see a much older face in the mirror.
I’m aware of Flepos but I’m not familiar with the details. This Fendt GNSS receiver would need older dual constellation RTCM messages like 1004 for GPS and 1012 for GLONASS. Flepos very likely has an option for this correction data content rather than sending the new multi-constellation messages. With the OEM modem you could easily have RTK accuracy supported.
My price information for that AGI-4 RTK modem is from about 2017 where my notes say 2400 Euro (I believe 0% VAT). As much as I’d prefer to promote AOG, at your starting point I’d say the OEM RTK modem is the only option that makes sense. Just need to find it at the correct price.
Even if the AGI-4 receiver can be used with an external RTK modem, it would not support RTK if the OEM modem was not present. If they have implemented something like RSA to verify the genuine modem, hacking the system to make it work without the genuine modem would cost more than the OEM modem.
You could try to find a second hand product from www.agriaffaires.co.uk or www.agcotrader.com or similar. I saw a complete receiver with a gyro and an RTK modem at 4000 € 0% VAT! The radio modem is different even for different EU countries but if your plan is to use RTK over NTRIP, any AGI-4 modem would be fine.