Android 8.0 is 5 years old. Google no longer supports security features on that version of Android.
One thing that Google has changed with the apps is you have to periodically go into the app and approve various permission settings. Google did this to make the entire Android platform more secure. However, it is a pain for users. If apps are used infrequently, you might need to reauthorize permissions. You might want to check the permissions. That might fix the issue.
I do not see anything on the app installation page that says you have to have a minimum Android version. Generally, if you try to install and app and it is not compatible with your device, it will alert you.
Is your Android 8 phone the only Android device you have. Cell phones can be quite expensive, but Android tablets accessing WiFi.
If you look at the reviews of the Traeger app on the Google Play Store, you will see there is a significant percentage of reviews with only 1 star. It is not a great app. Although I do have my grill connected to the app on one of my tablets, the only thing I find useful is alerts for the pellet hopper running low. I never monitor temperatures using the app as I use 3rd party thermometers to monitor both grill and internal temperatures.