Now do not care what you use on the house side of your router.
But I would not use a booster, I would use a pair of PowerLine Adapters, with WiFi at the upstairs end, £50-£60. Have a look at Curry’s website for a suitable one.
Or I might just get a better router. The Sky SR203 is easily and cheaply found on eBay, has much better wifi range, and needs no further configuration to replace the Now one.
Just keep the Now one to swap back in if you ever have to notify a problem to Now on your broadband.