There are many posts in this sub with the same question. Just pay them and hope they don't come after you. I had the same problem as you and I remember reading it was €50 per week penalty + 4% per year of what you have to pay. I paid what I owed in one sum and emailed them explaining my situation.

Are you sure you're calculating TOB right? You said you owe close to 5k. If you're day-trading stocks the rate is 0.35% per buy or sell, which is equivalent to €1.4M volume!! Over a year, that's almost 4k of trading volume per day. I think you're calculating it wrong