I had the same issue with my AP 7 and Dexcom G6 and tried out several solutions posted here:
1. Turn ipone (12 pro max) and AW off and on => no success
2. Reinstallation of Dexcom app on iphone => no success
3. usage of Dexcom follow function => no success, as this app doesn‘t offer an AW complication/app in my region
4. Sugarmate (currently out of service) and nightsout => no success, also no AW complication/app in my region
5. Registration for Beta on iPhone and AW and installation of watchOS 8.3 on AW => no standalone success
finally I noticed, that the AW obviously alway shows the „please check Dexcom app“ error, whenever my old AW5 is turned on and close to the iPhone (I didn‘t disconnect the old AW5 and connected the new AW7 as a 2nd watch.) This issue was reproducable, whenever the old watch was turned on and close, the Dexcom app (on both watches, also on the AW5, which now worked perfectly for 2 years) showed the error message. Seems like Dexcom can not judge on which AW the data shall be displayed.
When turning of the old watch, the Dexcom app on the new AW7 works, though there seems to be a 5 min delay, before data is updated on the watch sometimes, which wasn’t the case on the AW5. So this hopefully will be fixed some day.
So as a summary, the combination of these 2 measures was successful for me:
1. Registration for beta and installation of watch OS 8.3
2. bringing the old watch out of range or turning it off.