You can turn it ON in the router.
See -> Recommended settings for Wi-Fi routers and access points
Recommended settings for Wi-Fi routers and access points - Apple Support
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Auto-Join when used with wireless carrier Wi-Fi networks
Wireless carrier Wi-Fi networks are public networks set up by your wireless carrier and their partners. Your iPhone or other Apple cellular device treats them as known networks and automatically connects to them.
If you see ”Privacy Warning” under the name of your carrier's network in Wi-Fi settings, your cellular identity could be exposed if your device were to join a malicious hotspot impersonating your carrier's Wi-Fi network. To avoid this possibility, you can prevent your iPhone or iPad from automatically rejoining your carrier’s Wi-Fi network:
- Go to Settings > Wi-Fi.
- Tap next to the wireless carrier's network.
- Turn off Auto-Join.