Hello, friend!
I haven't heard of PIM router on MA5600.
You mean Designated Router?
Run the pim ipv6 hello-option dr-priority command to set the priority of the local device for DR election.
# Set the priority of VLANIF10 for DR election to 3.
huawei(config)#multicast ipv6 routing-enable
huawei(config)#interface vlanif 10
huawei(config-if-vlanif10)#pim ipv6 hello-option dr-priority 3
Thanks!