The post @erlinden linked is generally fine. However the setup has one drawback (which might be show stopper): it is not possible to directly access devices "behind" wireless device. As you're mentioning server (for which powerline adapters are not reliable enough), I guess you are actually after an L2-transparent wireless link which would make wired hosts in the attic appear full members of the rest of LAN. This is not possible when talking about pure 802.11 standard because standard lacks room for 4-th MAC address. Different vendors created different work-arounds and standard extensions and are mostly incompatible. So you will have to use wireless client (in the attic) by the same vendor as AP.
Now, if your AP is mikrotik as well, then setup of hap lite is easy ... but you have to do it from blank configuration:
- create a bridge and add all ether ports and wireless port to it
- configure wlan1 interface, set mode=station-bridge, configure SSID and PSK according to settings on your AP
- on bridge interface either set IP address statically (add default route using main router's IP address as gateway) or set DHCP client so you'll have admin access to hAP lite via your LAN