In a hurry, so let me know if anything is not clear.
CODE
# apr/12/2018 18:26:28 by RouterOS 6.42rc52
# model = RouterBOARD 750G r3
/interface bridge
add admin-mac=xxxxxxxx auto-mac=no comment=defconf name=bridge1 vlan-filtering=yes
/interface bridge port
add bridge=bridge1 interface=ether2 pvid=50
add bridge=bridge1 interface=ether3-trunk1
add bridge=bridge1 interface=ether5-wan1
add bridge=bridge1 interface=bonding1 pvid=50
/interface bridge vlan
add bridge=bridge1 tagged=ether3-trunk1,bridge1 untagged=ether2,bonding1 vlan-ids=50
add bridge=bridge1 tagged=ether5-wan1,ether3-trunk1,bridge1 vlan-ids=600
This is the bridge config on my main router, where Unifi is coming into.
My VLANs:
vlan600 for HyppTV.
vlan50 for LAN traffic.
There're 2 more vlans but can be ignored so I removed them from the above config. I don't have any untagged traffic.
Most important:
ether3-trunk1 is the trunk to my RB2011UAS downstairs. (trunk for vlan50 and vlan600)
ether5-wan1 is connected to the BTU.
ether2 is connected to a switch. (vlan50)
ether1 and ether4 are bonded to another Mikrotik. Can ignore this. (vlan50)
Downstairs, for the router on the other end of the trunk, you configure something similar to the above.
CODE
# apr/12/2018 18:40:00 by RouterOS 6.42rc52
# model = 2011UAS-2HnD
/interface bridge
add admin-mac=xxxxxxxx auto-mac=no name=bridge1 protocol-mode=none
/interface bridge port
add bridge=bridge1 interface=wlan1
add bridge=bridge1 interface=ether1
add bridge=bridge1 interface=ether6
add bridge=bridge1 interface=ether2
add bridge=bridge1 interface=ether5
add bridge=bridge1 interface=ether7
/interface ethernet switch port
set 1 vlan-header=add-if-missing vlan-mode=secure
set 2 default-vlan-id=50 vlan-header=always-strip vlan-mode=secure
set 3 vlan-mode=secure
set 5 default-vlan-id=600 vlan-header=always-strip vlan-mode=secure
set 11 default-vlan-id=50 vlan-mode=secure
/interface ethernet switch vlan
add independent-learning=no ports=ether1,ether5 switch=switch1 vlan-id=600
add independent-learning=no ports=ether1,ether2,switch1-cpu switch=switch1 vlan-id=50
Mine is like this. For educational purposes, my RB2011 is configured using the switch chip. So may bit a bit confusing. It still is to me.

Maybe easier if you just configure using bridge and vlans like the first router.
Note: Not sure 100% if all this is the optimal way but it works.