As @erlinden already hinted, VLANs are your friends ... you'll have to make ethernet cable between hAP ac and hAP ac2 trunk with two VLANs, one will be internet and the other LAN.
As the hAP ac features a very decent switch chip while its CPU is not very fast, you better configure VLANs on switch chip. I'll assume that sfp1 will be used for internet (untagged from ISP's side) and ether1 will be used as trunk port for connection towards main router. ether2-ether5 will be part of LAN in my example. In my example, VLAN VID=100 will be used for WAN and VLAN VID=200 will be used for LAN.
When entering the setup, connect to ether5, enable safe mode and enter the configuration. When dealing with VLANs it's too easy to lock yourself out ... and not even MAC winbox helps there as one locked self out on L2 level (in switch chip).
# Note: Don't try to set vlan-filtering=yes on this bridge, it's not that kind of setup
/interface bridge
add name=bridge # admin-mac=<MAC of ether1> or localy-administered MAC
/interface bridge port
add bridge=bridge interface=sfp1
add bridge=bridge interface=ether1
add bridge=bridge interface=ether2
add bridge=bridge interface=ether3
add bridge=bridge interface=ether4
add bridge=bridge interface=ether5
add bridge=bridge interface=wlan1
/interface ethernet switch port
# in principle one would have to use port numbers, not port names. I've no idea how are port numbers on hAP ac ...
# The code below might work. If it doesn't, find out the port numbers and substitute port names with corresponding port number.
set ether1 vlan-mode=secure # no default-vlan-id on ether1, it should be all tagged trunk
set ether2 vlan-mode=secure default-vlan-id=200 vlan-header=always-strip
set ether3 vlan-mode=secure default-vlan-id=200 vlan-header=always-strip
set ether4 vlan-mode=secure default-vlan-id=200 vlan-header=always-strip
set sfp1 vlan-mode=secure default-vlan-id=100 vlan-header=always-strip
/interface ethernet switch vlan
add independent-learning=yes ports=sfp1,ether1 switch=switch1 vlan-id=100
add independent-learning=yes ports=switch1-cpu,ether1,ether2,ether3,ether4,ether5 vlan-id=200
/interface vlan
add interface=bridge name=vlan200 vlan-id=200
/ip address
add address=<hAPac's LAN IP> interface=vlan200 network=<LAN network address>
/interface wireless
set [ find default-name=wlan1 ] vlan-mode=use-tag vlan-id=200
If you still have connectivity after entering the setup above, exit safe mode. Then plug your computer to one of ports ether1-ether4, reconnect to hAP ac, enter safe mode and proceed with this setup:
/interface ethernet switch port
set ether5 vlan-mode=secure default-vlan-id=200 vlan-header=always-strip
set switch1-cpu vlan-mode=secure
Proceed with configuring WiFi (SSID, PSK, ...).
The VLAN-related setup on hAP ac2 (main router) should be done
the new school way with vlan-filtering on bridge. I'll assume ether1 will be used to connect to hAP ac (so tagged trunk port). The rest of ports will be access ports for LAN ... Connect to ether5, enable safe mode and go:
/interface bridge port
set [ find interface=ether1 ] frame-types=admit-only-vlan-tagged ingress-filtering=yes
set [ find interface=ether2] frame-types=admit-only-untagged-and-priority-tagged pvid=200 ingress-filtering=yes
set [ find interface=ether3] frame-types=admit-only-untagged-and-priority-tagged pvid=200 ingress-filtering=yes
set [ find interface=ether4] frame-types=admit-only-untagged-and-priority-tagged pvid=200 ingress-filtering=yes
set [ find interface=wlan1 ] frame-types=admit-only-untagged-and-priority-tagged pvid=200 ingress-filtering=yes
set [ find interface=wlan2 ] frame-types=admit-only-untagged-and-priority-tagged pvid=200 ingress-filtering=yes
/interface bridge vlan
add bridge=bridge tagged=bridge,ether1 vlan-ids=100
add bridge=bridge tagged=bridge,ether1 untagged=ether2,ether3,ether4 vlan-ids=200
/interface vlan
add interface=bridge name=vlan100 vlan-id=100
add interface=bridge name=vlan200 vlan-id=200
/ip address
add address=<router's LAN IP> interface=vlan200 network=<LAN network address>
# add/move WAN config - static IP, DHCP client, PPPoE client, ... to interface=vlan100
/interface list member
remove [ find list=LAN ]
remove [ find list=WAN ]
add interface=vlan100 list=WAN
add interface=vlan200 list=LAN
After that, if you still have connectivity, exit safe mode, connect your computer to one of ether2-ether4 and proceed with this part of config:
/interface bridge port
set [ find interface=ether5] frame-types=admit-only-untagged-and-priority-tagged pvid=200 ingress-filtering=yes
/interface bridge
set [ find bridge=bridge ] vlan-filtering=yes
Then move whatever services for LAN from interface bridge to interface vlan200 ... those services likely include DHCP server and perhaps some more.
If your firewall is at default, which uses interface lists named LAN and WAN, then you're good. Check interface list membership for dynamic interfaces (such as PPPoE).
The above configuration example doesn't deal with complex wireless setups ... if you want to run multiple SSIDs (e.g. for guest access), you'll simply introduce additional VLAN ... and possibly change firewall filter rules to separate guest subnet from home subnet.