mikrotik 4011 is a great router and we keep routers' firmware up to date with long-term 6.43.14
we have noticed following issue in 3 routers.
- router is passing data traffic fine.
- we can ping the router fine.
- we cannot mac-telnet, ssh, or winbox into it.

--- not sure what is causing the issue but it fails backup then we come to know that router is locked up. we have replaced one but 2 of them are still in production and cannot reboot or anything. what would be the possible cause for it?
PS - usernames/passwords are changed and removed default admin user. Also ip firewall blocks any unauthorized ips coming into input chain. under ip services-- only winbox, ssh n api-ssl are enabled - everything else is disabled. please help!!!

You need to give a bit more for people to be able to help you.
Can you post an /export hide-sensitive so people can see the problem?