There are different variants why it stopped working properly. I've experienced kinda similar situation, however I'n not that smart in techs to try to fix it. So, I just bought a new one. By the way, I've had only brand models for the couple of years, however I decided to try buying something more affordable, but still qualitative. I've been browsing in searchings for the best budget variant when I came across
https://readyresearch.com/best-microwaves-under-100/ . This article consists of the list of best microwaves "under 100", the buyer's guide and even FAQ is provided for the common questions, so everything you need to know before making a decision is gathered in one useful article. I've bought Nostalgia RMO7RR Retro Microwave which matches all the characteristics it was given there. Just look at that red color! Perfect choice. I suggest you try to look through this page and pick sth with lower price. At least it wouldn't be as disappointing as if brand model breaks down, so just check it!