You likely roll for initiative, and certainly if the DM says so
In general your DM is right, you would roll initiative at the beginning of an encounter, and no attacks should be made before initiative. But there is an exception. The Core Rulebook pg. 498 has this option:
Initiative After Reactions
In some cases, a trap or a foe has a reaction that tells you to roll initiative. For instance, a complex trap that’s triggered might make an attack with its reaction before the initiative order begins. In these cases, resolve all the results of the reaction before calling for initiative rolls.
How would these foes set up their reaction to attack you? And conversely how could the characters, if they are the "foes" that set up an ambush for some other monster? There are special monsters that explicitly state they have reactions that happen before initative, like the hunting spider. The only way for the characters, who lack such special reactions, would be to use the Ready action, which sets up such a reaction. But is it possible to take that action outside of combat?
The rule on Actions on p. 461 Core Rules states that you can can use actions outside of encounter mode:
You affect the world around you primarily by using actions, which produce effects. Actions are most closely measured and restricted during the encounter mode of play, but even when it isn’t important for you to keep strict track of actions, they remain the way in which you interact with the game world.
Unfortunately, the Ready action talks about doing something outside your turn, which would imply that you already are in encounter mode and turn order, because a turn is only defined once your are under initiative order. From the Key Terms section for Turn:
During the course of a round, each creature takes a single turn according to initiative.
So this would rule out that you can ready an action on technical grounds.
While there are rules for surprise, they primarily deal with how you can use stealth to get a leg up on initiative. In this situation, the summoned creatures have no way to discover the characters before they appear, and therefore the rules for Stealth and surprise at the start of an encounter do not make much sense.
I think one way to resolve these contradictions between the Ready action and the Initiative after Reactions rule that makes narrative sense is to allow characters to Ready an action outside of combat, if the circumstances are right, and hold it until the start of the encounter, where it could resolve before rolling initiative.
So in your situation you could attack with your readied action, and then roll initiative. But because technically you cannot Ready an action before you are in initiative, in the end it is your DMs decision how they handle it. By default, you would have to roll intiative.