2100/300 plan -- Ubiquiti UCI Sees 2100 down consistently, but only 40 up

My plan is 2100 Mbps (down)/ 300 Mbps (up).

My modem is customer owned and is an Xfinity-supported model (Ubiquiti UCI).

https://assets.xfinity.com/assets/dotcom/projects/cix-4997_compatible-devices/2023.09.25%20Full%20List%20of%20Compatible%20Devices.docx-%20vF.pdf

I consistently see 2100 Mbps down.

However, I only ever see 40M Mbps upload.

It appears that others (https://forums.xfinity.com/conversations/your-home-network/ubiquiti-uci-nextgen-x2-only-40mbps-upload/678be8b9ec3cb6282638d323) have had similar issues and been able to get it resolved.

What can I do to get help checking whether my modem is properly provisioned for the speed tier I'm on, including upload speeds?

Thanks.