How to share internet via Wifi hotspot/USB tethering on Xperia SP?

I am unable to share my mobile internet connection via Wifi hotspot or USB tethering to a laptop. When enabling the Wifi hotspot, the laptop connects to the Wifi just fine, but whenever I try to access a website on the laptop it ends in an error; on the phone it works fine. Same issue with USB tethering: The laptop connects just fine, but internet itself isn't shared. Now I've isolated the issue as coming from the phone itself: * internet works fine on the Sony Xperia SP * same SIM card on a different phone: sharing internet connection works fine (the type from the phone company was adamant they can't block this from their side) * connecting phone to home Wifi and sharing that internet connection via USB tethering to a laptop works fine * using a different SIM card on the Xperia SP and I still can't share the mobile internet connection The only thing that comes out of it is that for some reason the Xperia SP somehow is incorrectly configured and doesn't share mobile internet access via Wifi/USB. Which settings should I check on the Xperia SP to ensure that sharing mobile internet connection works?