IPv6: You cannot enter a network IP for Gateway IP

Greetings,

Sophos XG is not accepting my IPv6 config while all other devices including Windows and Linux Hosts accepts.

i get error "You cannot enter a network IP for Gateway IP"

my ISP assigned config as below 

IPv6 POOL-2401:0:4000::510/126
Gateway-2401:0:4000::510
Uesable-2401:0:4000::511~2401:0:4000::513

any ideas