If you have a little patience Mendix will fix this for you and make iOS devices on par with Android in terms of offline database storage. I don't think they've communicated official timelines though.
I wouldn't recommend utilising the browser's inbuilt local storage due to security concerns.
Unfortunately Apple is not too keen on allowing webapps having the same capabilities as native apps, but with the possibility to receive push notifications from iOS 16.4 on, we are starting to see this change a bit. For now however, it's still early days for PWA's on Apple devices and you will have to work from the cache, rather than a local database.