Your IT Guy is right: if you want to see specific separated tasks in each channel, and not across all channels then you have to create a new plan in each channel. So when adding a planner Tab in a channel select add a new plan and do not link it to an existing plan. You may need good naming conventions for plans (I.e. aligning them to channel names) so it doesn't get too confusing (thinking here if there are lots of channels)
Yes, make sure you create a "sub plan", or plan using Teams, don't create a New plan in planner for each channel or you will end up with a ton of Groups :). However you do have the sub plan ability in Planner now, but you have to be careful when you click on New Plan, make sure to Assign the plan to an existing group (Your Team). And you can create these via Planner as well, but honestly it's easier in Teams to just go to the channel, add a tab, and New Plan.