If it's out of warranty, then even if you think it's a software bug, you're still dependent on them to correct that bug (I don't think the user manual tells how to do it). So, like it or not, you have to trust either Samsung customer support on the phone, or your local Samsung certified to solve your issue. So your next step might be a local tech and ask his opinion.
Hi Samsung :
My UHD U55 F9000 reboot loop , samsung support tell me replace main board , is there any other solution , looks like software bug