In-band - should always have at least SSH available if not a secure VNC connection.
Many rack-based servers also have ILOM/ALOM - basically a service processor that can be talked to via Telnet/SSH/VNC or a Web-Based applet
And then there's the console - the RS232 port, which again, in the rack, many servers will have the console port parked out towards the aisle side where it's accessible - and there - we have "crash carts" - which is typically an old laptop (think Dell 610 or similar Thinkpad) with a comm port and a cable if nothing else works...
We refused the KMM's - HP/Dell/IBM have them, as they're very expensive for what they do - nice to have one in a rack, that being said - and that's why I suggested eBay - as many data centers at the moment are in extreme turnover, so there's a lot of interesting gear out there...
(FWIW - one can get all Mr. Robot and use a RaspPI with a USB-Serial connector on one-side, and an ethernet connector on the other, just don't mess with the climate control and wreck the backups)