Good to hear! I hope I will be able to manage it as well in Kaliningrad this summer.
Can you confirm that you have been sold a prepaid simcard using the tariff "МТС Коннект-4"? That would make sense as they charge 1 rouble per MB:
??? ???????-4 - ?????-?????????, ????????????? ??????? so if you initially had 400 roubles that would indeed make 400MB.