Just had to comment on this thread because a fair bit of misinformation on the antennas.
The Inverted F antenna that is being modded in this thread is not in use.
It is certainly a 2.4ghz antenna and not for 868mhz, in the pictures by wandal and solbadguy2010 it is obvious that the 2.4ghz is not being used - the supporting components nearby are unpopulated - in fact on that modded board the modded antenna is not even connected to anything.
The helix antenna at the end of the board is the correct one that is being used.
here is the datasheet for the 868mhz antenna
here is the datasheet for a potentially better design
it also could be possible to increase the output power of the chip itself - the radio chip uses SPI for setting the output power https://www.ti.com/lit/ds/symlink/cc110l.pdf?ts=1654777792616
Either way, it's all great upgrading the bridge antenna but it could equally be problems with the device antennas
I don't think these devices can/will ever "mesh", but being able to add another bridge should be a relativly easy software "fix" if tado really wanted to sort this.
This version that was posted by solbadguy2010 (which does have chips to support the 2.4ghz antenna) and has a sort-of-UFL connector (see https://electronics.stackexchange.com/questions/88041/what-connector-is-similar-to-u-fl-but-has-no-pin)
Anyway, if you buy a 50ohm 868mhz antenna and adapter such as these and then you will need to modify the PCB slightly to use the external antenna and that should work a lot better than the built-in antenna