Playing With Mobile Data

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Because there's a lot more to a network connection than bandwidth. Connection latency and packet loss is a lot more important for gaming than raw bandwidth, and mobile hotspots/phone tethers in many cases have terrible latency and relatively high packet loss, depending on network type.

Is your mobile connection 3G or LTE 4G? Playing on most 3G networks is going to result in very bad latency, but LTE connections (in Europe at least) will probably work fine, with sub-50 millisecond delays. If you're getting 40 Mbps on downloads, it sounds like a 4G connection, but you might want to do some tests on pingtest.net to see how stable and responsive the connection is.

Also, are you connected to the actual hotspot/phone wirelessly or by cable? Wireless connections always add a chunk of extra latency to all network communication.

EDIT: Also, go to arrstatus.com and find out the IP address of your datacenter, then open Windows' Command Prompt and type tracert followed by the IP address, and post the results.