@J4k00: Where did you purchase the S20 Ultra from, and where are you currently based? The 'Global' network option seems to be a feature of US devices, therefore if you're based outside the US then this may be why you're experiencing this issue. You may wish to try heading to Settings > Connections > Mobile Networks > Data Roaming, and disable this option if applicable, to see if this helps. Also, try tapping on Access Point Names > Tap the 3 dots in the top right > Reset To Default.
@J4k00: I'm sorry to hear that. Although not ideal, I recommend reaching out to the seller to arrange a refund, as software and hardware variances in the device will create compatibility issues with some apps and services, such as those you've encountered with your Mobile Data connection.