Installing iOS on an iPhone 11 Pro Max

See this for help with over the air updates, If your iPhone, iPad or iPod touch won’t update - Apple Support

Here are the conditions that must be met to update iOS:

• You must have a fast Wi-Fi connection; you cannot update over cellular data
• The phone can not be in Low Power mode
• The battery state of charge must be above 50%
• You must have at least 3 GB of free storage
• You cannot be connected through VPN
• If you are not planning for it to update automatically you should turn this option off, as it will only update when the phone is plugged in, connected to Wi-Fi and locked
• You should leave the phone plugged in until it finishes updating, both to assure that it remains above 50% and to keep Wi-Fi enabled (Wi-Fi turns off when the phone sleeps if not connected to power).

It is not "impossible".

If you're having trouble, you're going to have to tell us what's wrong if you actually expect help.

from my experience best not to as it knocks out Bluetooth

GorillaB wrote:

from my experience best not to as it knocks out Bluetooth

It didn’t for me. Bluetooth works perfectly with 13.4.1.

You’re lucky - we have four pieces of kit all Been updated now none of them work