In Europe, everything uses 433.92 MHz and at power levels an order of magnitude higher than that allowed in the USA & Canada so I'm dubious that you have antenna issues. Wrapping some small diameter wire around the existing antenna without any permanent changes should tell you whether that's the case.
We do not see many poor RF range complaints from Europe because of the much higher power allowed but there may well be interference issues since everything (ceiling fans, garage doors, lights, etc.) use the same frequency.
