Poor RF communications : looking for advices
A repeater for that system would probably have to be protocol aware (ie made by Honeywell). A simple repeater arrangement that tried to receive at 868MHz and retransmit would introduce a slight phase delay. If the receiver is already receiving the original transmission most of the time that would likely confuse the data demodulator.

You could try pulling them apart and seeing if fitting a larger antenna to one end is possible. Depending on the original antenna something like a small 1/4 wavelength whip (around 8.5 centimeters at that frequency) may double the range. Improving the antenna increases both the transmit and receive gain, so just changing one end of the system may be enough and is worth trying.

Note that the various regulations on RF emissions are based on radiated power, so doing that may put it over the approved limits for license-free devices in your region.