I have a scenario similar to the Mikrotik dhcp relay wiki page: A main router of another brand that makes the connection to ISP, this is also main DHCP server (192.168.0.1).
To it, I connected a Mikrotik HAP ac2 (192.168.1.1)

Now, I want a computer that is plugged to Mikrotik eth2 port, to use a static IP from upper router , e.g computer use IP 192.168.0.5
On the computer I manually set gateway to 192.168.0.1 and ip 192.168.0.5, but ...

- I understand the Mikrotik needs to be setup as a DHCP relay ?
- I thought it might work directly since on the computer I manually specify upper gateway 192.168.0.1 and static ip 192.168.0.5
- That computer is only accessible once I add the eth1 to the bridge (like in image), is this correct / needed ?
- Why doesn't that bridge already contain the eth1 by default, but it contains all the other ports ?

This seems to be exact same situation and solution: as this topic says.

Problem solved everything works.
I added ether1 (WAN) to the bridge used for DHCP-relay.

That won't work, the wiki example is using DHCP relay to pass requests for 192.168.1.0/24 and 192.168.2.0/24 subnets to a DHCP server running in the 192.168.0.0/24 subnet.

If you wish to extend the network from the other router the hAP should have all ports in one bridge, no DHCP server or DHCP relay. The hAP itself can be assigned a static address or use the DHCP client to obtain one from the other router. I don't think there is a Quickset option for a bridged AP so it is a case of removing the unnecessary bits from the default configuration.