Can't update iPhone 15 Pro Max to iOS 18.3 over 5G network

So I have an iPhone 15 Pro Max running IOS 18.2.3. This morning I read that there was a zero day exploit and I should update IOS as soon as I can. So I opened the settings app, tap on software updates, and sure enough there's an update to 18.3. The big issue:

It says I need wifi or 3G or Faster network. But as seen in the picture, I'm on 5G (TPG Singapore aka Simba). In the end I needed to create a Hotspot on my other phone and then tether my iPhone to that phone- an extremely weird solution!

Why does IOS not allow me to update the phone over 5G?

[Re-Titled By Moderator]