Sounds like the STB only has RF pass-through i.e. all it does is allow you to share the off-air transmitted RF signal with another device e.g. a TV RF input. Trouble is, when you share an unamplified RF signal like this each device is only getting part of the RF signal. The portion now being fed to your TV card's RF input may be below the strength needed to tune successfully. In other words the STB is doing nothing for you except sucking out a chunk of the aerial/RF signal strength!
If you want to use the RF input on your TV card you need an STB that is not just pass-through but outputs modulated Freeview programming via an RF output. The early STBs did this but it seems to be uncommon nowadays.
An alternative would be to check the manual to see whether the STB outputs a composite signal via SCART and connect this to the TV card's composite input (most have this) via SCART to Red/White/Yellow phono connectors.
Better still, get your money back on the STB, junk the TV card and get yourself a USB Freeview dongle for about the same money!