Congrats on the purchase.
I don’t think this is fuel efficiency but is estimated range?
The car uses a calculation of the fuel used over the last ~30 miles to estimate range of the rest of the fuel in the tank.
If the previous ~30 miles were a motorway run, and then you do short / cold start journeys it will drop quickly and then stabilise.
You mention that mpg is low but what does the screen say is your fuel consumption? As you have just bought the car it would be worth resetting the long term average and keeping an eye on it to see if you have a problem.
If it is dropping quickly then have a mechanic check it, or talk to the garage if you bought it from a garage.
Good luck.