assuming you have iphone and apple watch you should install sugarmate and it should work fairly well.
If you are indoors and both phone and watch are both connected to WiFi you should see your BG without carrying your phone. Apple support article here.
If you are outdoors with WiFi e.g. small garden close to your house it may work, or you can buy a Wifi extender and put it close to the nearest window to the garden. You can also put the phone on the window still against glass (outside) to connect to your indoor Wifi and it may stay connected.
If you are outdoor out of range of your home Wifi you can try put your mobile phone flat on top of an empty glass or stand it up against something like a mug or bottle. This will give you the best chance of staying in range. lying the phone flat on a table or keeping it in a bag won’t be as good. The Phone cover may also lessen the range so you could try remove the cover for this particular activity.
In summary the idea is to try keep the phone connected to WiFi and then keep the watch in range (30ft-100ft) of the phone. Glass and air are good, walls, purses and thick phone covers are not so good.
The same basic principles may or may not apply to other phones and watches, if you post the make and model of both you may get more specific advice
Ed