How to Sync Network Priority From Mac to iPhone or iPad
You'll be glad to hear there's a sneaky way to get around the lack of a Wi-Fi priority setting in iOS. If you own a Mac, you can set network priorities there and then sync them with your iPhone or iPad.
You'll need to have signed in with the same Apple ID on both devices, and have iCloud Keychain turned on.
To check these settings, open System Preferences on your Mac, click on Apple ID, and make sure that the box for Keychain is ticked. On your iPhone, open Settings and tap your name at the top. Go to iCloud > Keychain and make sure it's toggled on.
As long as you've got both of those settings turned on, you're ready to sync your Mac's Wi-Fi priority settings to your iPhone.
On your Mac, click on the Wi-Fi icon in the top menu bar and then click Network Preferences. Next, you'll need to click on Advanced.
You'll see a list of all your saved Wi-Fi networks. The networks at the top of the list have a higher priority than those at the bottom, and you can drag networks into the order you want. Click OK and then Apply to save the changes.