The shop has given poor advice I'm afraid as NO2 (nitrite) should be 0 and nitrite is very toxic. Nitrate (NO3) ought to be around 40ppm or so and they haven't tested for ammonia which is critical. If you have nitrite (NO2) in the water you probably also have or have recently had ammonia. The ammonia and nitrite are the reason for the fishes' behaviour. You need to change at least 50% of the water (preferably more ) to get those levels down. But check your tap pH as the pH in the pond is low and if the pH in your supply is higher by much, then the water change could shock the fish so it would need to be done as multiple smaller changes gradually building up to a big one. Get a liquid based test kit that includes an ammonia test (API Master test kit is good) so you can monitor daily.
Check your tap water. If there's no nitrite in the tap water then water changes must bring it down, simple dilution - but you need to change something like 75% to see a difference. Water changes will not adversely affect the cycle as the beneficial bacteria are in the filter, not the water.
Have you added ammonia? If not, that's why there's no cycling. Pellets do't break down into ammonia readily - they are designed not to in order to avoid them polluting the tank.You need to add ammonia to simulate fish waste, and then the good bacteria that process that into nitrite and then nitrate will start to grow and deal with it. This article explains and has an ammonia dosing calculator at the bottom: https://www.fishkeeping.co.uk/articles ... hless-cycling-article.htm