The package you are on doesn't offer free "pods" to extend the wireless range. You can phone up and (if there's stock available, which has been a bit of a problem) pay £5 a month to rent one. But that adds up, in 18 months that's ninety quid for something you don't own. You're better off as @lotharmat says to invest in your own wireless kit. Personally I'd suggest that a mesh wifi system would be a better bet than a standalone router for your circumstances, and (so long as you don't intend to upgrade beyond 100 Mbps) a Tenda Nova MW3 (£63 from eBuyer at the moment) is a sound choice, but limited to 90-100Mbps.
If there's a possibility you might upgrade speed in the future, then something like a TP-Link Deco M4 or S4 is a good choice.