If the addresses are static they should provide a WAN /64 with both their end (the gateway) and your end addresses - the latter should be the target of the routed /56 addresses. If they are mistakenly just presenting a /56 on the WAN that will not work as it requires ND proxy as a hack which Mikrotik do not implement.
You appear to have swapped the /64 and /56 in the address and pool definitions.
Don’t use something:: as an IP address, any subnet all-zeros address is reserved for the ‘all routers in the subnet’ anycast address.
You can not guess what it should be, the ISP should provide it as they will be routing the block of subnets to it.
Their terminology is rather vague too - ‘IPv6 address’ does hints at being the WAN address but would typically be /64, not /56, and ‘routing prefix’ hints at the routed subnet but would typically be anything from /48 to /60, not /64.
Do they not support automatic configuration with SLAAC (for the WAN subnet and gateway) and DHCPv6 (for the delegated prefix)?