Yes?

Yes!

Try it and find out?

Yes it does.

This must be a post I’m too iPhone SE to understand.

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.

Great explanation — just to add FWIW, the second-gen SE supports it, too.

Why would they give you a feature that doesn’t work?

No Apple put it there because the toggle looked nice.

Barely, painfully slow at least.

I'm sorry... my explanation post didn't make it up.

I have an iPhone 13 Pro with a both a physical sim and esim from different vendors. Both have unlimited plans. One of my plans has terrible coverage outside my city and it never seems to switch to the other.

Has anyone else had this work? Are there other settings I should look at?

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.

No

Yes! I had AT&T on one SIM for my personal number and Verizon on another SIM for my work number. But I had all data go through Verizon, and would forward my personal number to my work number when traveling internationally as work would flip the bill for international calling. It worked very well and was something I rarely had to deal with once I set it up.

Eventually I changed the AT&T personal to Verizon because, well, I hate AT&T.

And when I left that job, Apple was able to transfer my personal sim to a new phone and my personal contacts went with it and my business contacts stayed on that phone which, I believe is more a function of iCloud.

Same, I’ve this activated, I also live in a area with poor coverage of one network but it’s good else where but it never switches automatically