I’m planning to setup zwift in my outbuilding, about 50 metres down my garden. It is out of the range of my wifi. Just wondering if running Zwift on a laptop using a phone hotspot would be ok, or would I need a faster connection to have full useability? Anyone done this? Many thanks.
Yes it is possible, though you may not be able to use the same phone to run Companion.
If your outbuilding is connected to the same electrical system as the location of your network equipment, you could also try using powerline Ethernet.
Have never tried it myself. Cat 6 Ethernet has a specified range of more than 50m, the 500MHz variant even 100m if I read correctly. I don’t know though if you need corresponding network infrastructure like hubs/switches to make that work - the range will likely not only depend on the cable but also on the signal transported over the cable. Please share your experience if you try this.
I have an Orbi mesh network with 1 Gb internet bandwidth that provides flawless coverage throughout the house, including two home offices. However, for tasks where managing latency is critical, such as Zwift and my work-related activities (the other way around actually lol), a wired connection is unbeatable (and way cheaper), so most of my devices are connected via Ethernet. Ideally, the Wi-Fi would only activate if the Ethernet failed, though that has never happened so far 
Note: I have tested the Orbi mesh network, and it works flawless up to a 200 yds from the house; in fact, I keep having relievable internet all the way to subdivision’s gates that’s crazy! Also, amazing for those weird places like under the stair’s bathrooms, garages that seems to be wifi dead spots. I have used my Verizon hotspot on vacation, and it isn’t reliable for a Zwift ride. Once in a while drops… maybe yours, it’s a lot better.
My companion app works ok through wifi but streaming the whole game from a gaming PC in the house over wifi to a laptop was stuttery.
I just use a powerlink adapter. That means I could also put a wifi booster in there for the companion app to use but so far I haven’t needed to.
The bottom line is, if you can get a wire there, whether it’s ethernet, powerlink or whatever, it’ll likely work more reliably than wifi.