What worked for me with the sync module was put your phone in AIRPLANE MODE when you first add the sync module to your system. I am using iphone 7 with iOS 12.1 with the most updated blink app and did have to update firmware on the sync module. Make sure you update your firmware more than once even if it says it was successful. Took two tries for me to get the actual firmware update.
I was able to update firmware and tried at least 20 times to get my sync module to connect until I tried airplane mode and connected to my home WIFI and connected as it should.
Gotta say, tech support just sent me another sync module and I sent the original back. It had the same issue and the firmware wasn’t updated when I received it. When I updated the firmware I noticed that you had to manually connect to the sync module whereas before the update it would pop-up a message and connect to the sync module. So make sure you are on your WIFI. Once you enter the serial number of your sync module it will tell you to manually connect to the sync module via WIFI. After you connect to your sync module it will ask for your home WIFI password so that the sync module can connect to the internet. After that it should connect to the Blink servers. Won’t work for everyone but this is what worked for me.
Shame that we are resolving these issues ourselves rather than an firmware update that will actually help solve these issues. And also there have been days when Blink customer support are not even working. Thanks for extending our stress and frustration.