I’m pretty sure it’s illegal in the European Union, if your employer requires this they will need to provide the phone, laptop or tablet. Its also not allowed for employers to make you install an app on your personal phone for example.

Its even more not allowed to monitor network traffic, then you will be violating both employee rights and the GDPR.

Dutch source: https://www.everphone.com/nl/blog/scheiden-van-gegevens-bij-mobiele-telefoon-van-de-zaak/