My new PCB design is about 25 mm * 25 mm, this is not enough for a proper 868 MHz antenna ground plane, I am using the 0868AT43A0020 chip antenna where at least a 20 mm * 40 mm GND plane is required. The first prototypes confirm the weak RF performances. what is the standard practice when it comes to dipole antenna when the ground plane of the PCB is extremely small and not sufficient?

A picture of the PCB with the battery behind: