Recently, a couple of clients had this issue with their Windows 10 notebooks. They were automatically switching over to mobile broadband/WWAN network in locations where wifi reception was poor. This was not the preferred option for their office environment because staff were familiar with controlling mobile broadband manually (and the associated costs!).
We found the option: Settings > Network & Internet > Cellular > Use cellular instead of Wi-Fi (shown below) – Disabling this setting had the desired outcome. But there was no matching group policy.
A little time with procmon highlighted that it’s controlled via the following registry setting:
0 = Never
1 = When wifi is poor
2 = Always
You can apply this across your fleet by using a custom Group Policy Preference.