iPhone 5 update before November 3: No new update available, but I don't have the necessary version

Hi people, I have just found out about this update https://support.apple.com/en-gb/HT210712 which says if I don't update my iphone to software version 10.3.4. lots of stuff will stop working on November 3rd 2019.

When I check the software version in Settings, it gives 10.3.3. - so apparently only a small update away. But when I check for updates in the next Settings tab, it says there are no updates available -- so I cannot update to 10.3.4.

The phone is not my daily phone but I use it for 2FA and payment apps, so it gets used once a week or so and I've kept it updated until now. I'd like to avoid having to move everything to my daily phone. What can I do?