Hello, I decided to report back here. After my last post my operator (Saunalahti) called me and offered 4G at the same price. I got (requested for it) new sim card which can be slit into smaller formats as needed (at least micro and nano). Changing the sim card did not have any effect, I was still able to send mms, but didn't receive them. Two (or maybe three) weeks back my daughter send me a picture and N900 got it without issues. Daugter's phone is Samsung Galaxy S5 mini. After that, I have received several mms pictures with success. I haven't changed anything. I suspect the the operator has changed something so that older version is also supported.

I have tried to make some inquiries, but have not found out any reason for this miracle recovery.