You can send a support ticket at
support.opensprinkler.com
and ask for a version of the OpenSprinkler logic board that has external antenna plug. It will come with ESP-07S WiFi chip:
https://www.rfsolutions.co.uk/downloads/6253e9bcb090a456DS-ESP07S-1.pdf
and you can buy a WiFi antenna yourself and plug it in. Please note that ESP-07S has no PCB antenna, which means it must use an external antenna, otherwise it won’t be able to connect to your router at all.
Other than the WiFi signal reported as poor/unusable, are you experiencing problem connecting to the controller? If you can still connect to the controller, I don’t think you need to bother with the antenna.