I believe ports 1-5 are in one switch group and 6-10 are in another. To use ports 3-10, you could configure ports 3-5 to one switch group, ports 6-10 in another, then bridge those master ports together.
But then a dedicated switch is probably better…after all the RB1100AHx2 is a router
/interface ethernet
set 0 arp=proxy-arp name=ether1 speed=1Gbps
set 1 master-port=ether1 name=ether2 speed=1Gbps
set 2 master-port=ether1 name=ether3 speed=1Gbps
set 3 master-port=ether1 name=ether4 speed=1Gbps
set 4 master-port=ether1 name=ether5 speed=1Gbps
set 5 name=ether6 speed=1Gbps
set 6 master-port=ether6 name=ether7 speed=1Gbps
set 7 master-port=ether6 name=ether8 speed=1Gbps
set 8 master-port=ether6 name=ether9 speed=1Gbps
set 9 master-port=ether6 name=ether10 speed=1Gbps
set 10 name=11_Public_11 speed=1Gbps
set 11 disabled=yes name=12_ether_12
set 12 disabled=yes name=13_ether_13
/ip pool
add name=pool_1 ranges=172.16.11.2-172.16.11.126
add name=pool_2 ranges=172.16.11.130-172.16.11.254