The internet-of-things that aren't really your things strikes again. When will manufacturers learn that their hardware should function even if their services are down? The whole account/subscription tacked on for no apparent reason thing is highly irritating, and Garmin is actually better than most.
When I worked for Garmin, we deliberately designed products to work for as long as possible offline - the Bike Speed & Cadence Monitors (~2018) are able to work for 1 year, riding every day, then a battery change, and then another year, all before a sync to a device is required. All of the rides should be stored on-device.
(I’ll accept “CA certificate expires” - but why isn’t there a workaround?)