Bandwidth and frequency are very different. They have a 54MHz wide uplink, at whichever of the supported bands you want.

I'm a person who knows nothing about these things, and my quick googling didn't help me - how "fast" is 54MHz wide uplink? Can it be measured in internet network speeds, or in what kind of way can I compare the speed of it?

It depends on the SNR. You can assume that almost all satellite connections use between 0.2 and 3 bit/s/Hz. It's likely closer to 1 bit/s/Hz though.

So operating with 54MHz of bandwidth would result in a transfer rate of somewhere on the order of 54 Mbps?

Many cable channels are using DVB-S2 8PSK at 5/6 rate. That would be 110 Mbps in 54 MHz (45 Msyms/s).