I tested it with a recent DELL R310 which doesn’t have PS/2. It booted from CD and I got into the selection menue, but I can’t get out of it.
Will this going to be supported?
Dunno for sure, but try enabling legacy usb support in the bios. This makes the USB keyboard emulate an 8042 port.
Unfortunately this Dell R310 BIOS doesn’t have such option. For this machine USBHID is necessary.
Further it also seems that I can’t give the image boot option to use e.g. the serial port. Or I’m mistaken?
I have problems with a RouterOs5.12 R310 to install … I get error when I try it from the CD.
Can not find the disc.
Posting multiple times in multiple forums will not get you a faster or better response. 
Post just the one time in the best forum after some careful thought or create a whole brand new post. Then be patient and see if there are any replies. Also, you are not very clear in your posting as to what exactly the problem is. What disc? HDD or CD?
if CD install doesn’t work, use Netinstall and boot from network. Surely that Dell can do that