Question [Q] Mobile connection drops and never goes back

Brand new xperia z1, bought 2 weeks ago in HK, used there for a 10 days with no problem.
Came to japan 2 days ago and now phone drops mobile network after 10-12 hours of stable connection. cannot reconnect, no restart helps. sim card works ok in other cellphone.
Upgraded firmware yesterday to newest one, but with same result - it worked for few hours, but by morning connection to network was lost and cannot connect back.
radio band choice in service menu shows only US band available and even then cannot select it (menu pops up back after selection).
anyone faced similar problem?