Need Help Connecting RV 890 to MacBook Air Running BigSur 11.3.1

I recently purchased RV 890 GPS for my new RV.  I want to find a way to plan a trip on my MACBook Laptop (and my iPad/iPhone if possible) and send route to my Garmin GPS.  I was told by Garmin to use BaseCamp Mac since you can't ent gpu files from Mac to Garmin easily anyway.   I installed the Basecamp software on my MAC and bought a USB cable to connect Garmin to computer directly.   I try to plan a trip in basecamp, but IU can't get very far.   Get message to connect GPS device to computer to get maps.   I can't find a way for computer and Garmin to recognize each other so that maps are available in baseCamp,   It's probably a setting somewhere, just not obvious to me.   Help anyone?