Welcome to the forum. NOW TV use adaptive bitrate management when streaming, and most devices will try to draw as much data as your connection allows to get the best picture possible (up to 720p) which will result in anything up to around a couple of GB of data each hour if you have a good 3G/4G connection on your Homefi. Unless you have unlimited mobile data you're probably going to find this uses up your data allowance very quickly and could end up costing you a lot in data charges.
Unfortunately NOW TV doesn't have any options to reduce data usage or select 'mobile' data streams so it may well be that you need to consider restricting NOW TV use.
It does have an option (cancel subscription) until these companies offering entertainment and the data options without a phone line can work together to offer a good service to its customers absolute joke!
Ok so this is probably going to sound bonkers but I THINK that I've found a way to cut data usage from a mobile hotspot by up to half.
I could do with someone else trying it.
I use an Amazon firestick lite but I imagine that the standard firestick is similar in that you can check the mobile hotspot connection in settings/network.
Anyway, I've noticed that if the phone is as far away from the firestick as possible (for me approximately 10 feet with a solid wall between) the signal quality drops from "very good" to "marginal."
The streaming quality drops.
Ive tried this twice on episodes that previously took about 1.2gb per episode.
Test one: 866mb.
Test two: 620mb.
I could do with someone else trying it.