The firmware can already accepts BSSID. At the WiFi configuration page, you can see it has an input for BSSID, which is automatically filled when you select a detected network, or you can manually type it in, or you can leave it empty so it will automatically connected to one of your routers that share the same SSID and password
To be honest, to address your issue you might want to consider having a secondary router that’s dedicated to OpenSprinkler, with a different SSID than your main network. Or, you can use wired Ethernet connection which can eliminate WiFi issues completely.
