For this setting to work your first plan needs to have no coverage at all, that’s what makes this setting so frustrating. What you can do is change the to the better line on the same page as this setting. That’s what I currently use: Primary eSIM for my actual phone number associated with iMessage, second physical SIM for DATA with the switch enabled.

That’s a feature when you have an iPhone with 2 active cellular plans (Dual Sim is available on iPhone XR, XS, 11, 12, 13, 14 and SE 3.)

The way iPhone handles Dual Sim does not allow Data to be active on both plans at the same time (that means the iPhone uses can receive and send calls on both sims all the time but uses the data plan of only one, even if the second one has better reception).

The feature OP asks about, allows iPhone to switch to the second data line if the first one fails due to bad coverage.