The maxi rider comes with a 5 point harness. When it's used as a booster seat down the track you don't use the 5 point harness, but you can get an anti-submarining clip instead, so there is still a strap between the legs.
At that age, you most definitely need to use the full 5 point harness - it would be extremely unsafe without it. Perhaps you could go back to where you bought it and get them to help you with it? My two DSs are in maxi riders at 4 1/2 and nearly 3, and the 5 point harness still fits very easily.
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