When it comes to keeping basements dry, there’s nothing that beats a sump pump. However, if it is set up to pump the water into the city sewer system, you have two problems.
First, the city does not want your water filling the sewers during storms. When that happens, either the sewers back up into peoples houses or the overflowing sewage ends up in streams, rivers and lakes.
Second, one of those homes into which the sewers may back up is yours. You will want to minimize the chances of that happening by disconnecting your sump pump from the sewer.
You’re probably not going to get away from this situation without bringing in a licensed plumber. However, considering the alternatives, it’s not such a bad idea.