When you changed to Rpi 3 b+, did you install Raspbian from scratch and install firmware by following instructions here?
https://openthings.freshdesk.com/a/solutions/articles/5000631599-installing-and-updating-the-unified-firmware
If you are using an older SD card with the original pre-configured SD card image, that won’t work because that was based on a very old version of Raspbian and it doesn’t support the newer RPis.