How Much Can A Bank Charge For Late Fees On A Mortgage Loan?

I made a mortgage loan payment after the due date which the bank charged me a $50.00 late fee. That is cool. I expected it. They claimed I skipped a payment because I went past 30 days between payments. They wanted a double payment at which I couldn’t swing. The late fee rose up to $150.00 a month and now is up to $254.00 a month. My payment is $500.00 a month. I paid them $1000.00 in July to bring my payment up to date and to meet my monthly mortgage payment requirement for July. However, I did not pay the late fee in full. Many banks will tack on the fees to the end of the balance. My bank is not and is charging me $254.00 late fee on top of a late fee. Is this legal? Are there limits to what they can charge? One late fee has grown to $550.00 for the month which is more than my regular mortgage loan payment. This makes it impossible to catch up. I tried to work with my bank, but the right hand does not know what the left is doing. Any suggestions? This is maddening! Help!