Mac not allowing iPhone hook up

Hey, when I connect my iPhone 11 Pro to my mid 2015 mac (running Big Sur) and look for the connection in the finder, it just shows it connected then disconnected just as fast and won't stop connect & disconnecting. I've ruled the computer to be the problem through the use of other cords and other macs. everything is running the latest software. does it on both USBs. I'm unable to back up my phone or any of the contents! Help! THANKS