Hello Schroe,
Thank you for using Apple Support Communities. Based on your post, you can't trust your Mac to sync your iPad mini. We'll do our best to assist.
It sounds like it's worked in the past. Other than upgrading to Mojave, any other changes to your Mac? If you're using a VPN or security software, we'd recommend to disable them and restart the device, as these may be blocking the connection: Check VPN (Virtual Private Network) and third-party software to help resolve network connectivity issues
Have you tried all steps in section, 'Get Help' here: About the 'Trust This Computer' alert on your iPhone, iPad, or iPod touch
Do you have another device you can test connecting to your Mac?
Have you tested another cable to connect the iPhone to the Mac?
Hoping to hear back, take care.