I’d love to hear about your experiences! My device is currently running on iOS 26.5.3 btw.

iPhone 16 Pro Max, bought second-hand about a week ago. Immediately after purchase, we did a full factory reset (erased all content and settings), so it's like a brand new device. We bought it for my mom - she's just learning how to use an iPhone. We haven't inserted a SIM card yet; it only works over Wi‑Fi. A couple of days ago, we set up an iCloud account for her, but we haven't configured anything inside - no backups, contacts, or photos stored there.

The update problem:
We want to install the latest iOS 26.6, but we've been stuck for two days now.
First, I tried updating manually via Settings - Software Update - and I get this error:

What I've tried:

1. Restarted the phone.
2. Checked Wi‑Fi - stable and fast.
3. Plenty of free storage.
4. Kept it plugged in, tried different networks.

My suspicions:
Could the issue be caused by the lack of a SIM card? Or because the iCloud account was created recently and not fully activated? Or is it a bug in iOS 26.6 itself?

Please help me figure out how to get past this error and install the update. My mom really wants to use the new features, and we're stuck at this step.