I’ve heard a few complaints regarding 3 but I’m on EE with no problem. I image you can go to a store and test out alternative networks, they surely have a SIM card you can try.
As the phone is working basically, there is no reason for a refund, I guess.
But are you really just deactivating VoLTE or enhanced LTE, as this should not disturb the LTE connection for data. Or didn’t you find this option?
You need to complain like mad to Three as well, they need to add support for each phone which seems a pretty crap set up if you ask me. I’ve complained to Three and Smarty (who also use Three’s network) about his issue the problem is I’ve no idea who has to configure what to support this.
4G does work with VoLTE and Enhanced LTE mode disabled, however it will drop to 3G for voice calling and 4G data connections will lag like mad randomly.
If it will not work again, so you can clear cache and perhaps the memory of the phone.
After Reboot on my Phone it was working agin.