Good choice in your router, I’m a MikroTik fan as well. Have you changed the username and password and also disabled the stock admin account?
Make sure it’s fully updated. Next, check for open ports on your router. If it’s close to a stock install, there should be a good half dozen ways someone can get in from outside.
Next, I’d check your firewall rules. You should have a “drop all from WAN not DST NAT’d” rule, if not, create one and that should be your last rule.
Mikrotik is extremely popular and is used worldwide by many, which also has its cons. There are bots that will brute force attack your router until they get in. It happens and it sucks, but use it as a learning lesson.