But returning to OP question,
what RouterOS version you have on RB960PGS?
the device chooses random ip
no, not choice "random" ip, it take one from the DHCP Server pool on main router.
1) Check if you are able to enter inside the config with WinBox using MAC address and not the IP
2) Full reset the RouterBOARD on System / Reset Configuration selecting only "No Default configuration" and "Do Not Backup"
3) After reset paste this script on WinBox terminal:
Place the correct IPs on this script:
/int bridge add name=bri-lan protocol-mode=none admin-mac=[/int ethernet get ether1 mac-address] auto-mac=no
/int bridge port
add bridge=bri-lan interface=ether1
add bridge=bri-lan interface=ether2
add bridge=bri-lan interface=ether3
add bridge=bri-lan interface=ether4
add bridge=bri-lan interface=ether5
add bridge=bri-lan interface=sfp1
/ip address add interface=bri-lan address=192.168.8.55/24
/ip dns set servers=192.168.8.1
/ip route add gateway=192.168.8.1