Hello Dirk,
Unfortunately, I don't have any practical experience with the Webasto FFB.
But the specified radio range is twice as long as that of standard DECT phones (300m), at 600m. In reality, most radio phones fail at around 30m. Therefore, you are already achieving more than double the stated range with 75m. It is unlikely that it will increase much further. The stated 600m applies under optimal conditions, i.e. in good weather and with a clear line of sight between the transmitting and receiving antennas, without any glass, metal, or obstructions in between. Probably even without reflections from nearby houses, hills, etc. Therefore, they are not achievable in reality.
What you can do to increase range, if the receiver's design allows: Place the antenna as straight as possible and as "externally" as possible, i.e. without any shielding plates around it. Either use a truly external antenna (Optics? No thanks), or place it in the upper area of the vehicle, where there is less metal, such as the engine block, to shield the radio waves. (e.g., on the upper part of the windshield, horizontally?). Then touch the transmitter in such a way that you don't shield the transmitting antenna with your hands. But I don't expect to double the range with these measures. And does it even make a difference?
My suggestion: install an old mobile phone with a prepaid SIM card in the vehicle, utilize the vibration alarm, and use a timer to activate the STH (presumably a stolen vehicle recovery system). This works perfectly over hundreds of kilometers. Just make sure no one reveals the STH mobile phone number

If the SIM card could be recharged, it would be perfect. However, some mobile phone contracts offer free partner SIM cards with their own phone numbers

That's how I do it. So far, I only get about one call every two years, and the person dials the number (I can see it in the call log). Then the STH goes off once every two years, and I'm okay with that.
Greetings,
Guste