iCloud photo library upload paused - will not resume

iMac 21.5-inch, Late 2012

2.7 GHz Intel Core i5

macOS High Sierra 10.13.5

Running out of space on my iPhone 7 Plus 128 GB, so decided to finally turn on iCloud photo library and upgraded my iCloud storage to 2TB. Photos (on Mac) now says:

99,144 Photos, 2,995 Videos

Upload Paused for 102,271 items

Not Connected to the Internet

Internet is fine, using web browser, email etc. I've tried closing and re-opening Photos, and have checked in iCloud settings which says:

9282 photos and 457 videos stored in iCloud

Your photos and videos are using 24.3GB

My Photos Library file on my iMac is 471.36 GB so I know not everything has been uploaded, and imagine it will take some time so I would like it to start again...how do I get Photos to resume? On my Photos app on my iPhone 7 Plus, I can tap "Resume" but on the Photos app on Mac it's not giving me that option.

Please help!

Thanks

Nick