Single post in Add Warning to PokéRadar Tab

You can't effectively chain without one or the other. The Radar on it's own can only 'chain' to a maximum of 1 which a chain of 1-3 technically isn't considered a chain and can be broken without warning. Both Radar descriptions pretty clearly state that you either need HM or the PR+ upgrade in order to actually chain with the Radar so it should already be clear enough that your chain will break if you hatch an egg in general.