»www.tp-link.com/Resources/docume ··· uide.pdf
At your 'main' office you want an AP, so configure as on Page 14 of the PDF, page 8 of the manual.
At your 'remote' office you want client mode, so configure as on page 15 of the PDF, page 9 of the manual.
To configure software, on Page 34 of the PDF both units are in Standard AP mode.
The 'main' unit is configured as an Access Point as on page 25 of the PDF
The 'remote' unit is configured as a Client as on page 29 of the PDF, I prefer to identify the AP by SSID rather than MAC address so if hardware changes, the configuration need not change.
If it was me I would configure with no security until I was sure I had it perfect, then add WPA2 PSK security. I would use WDS.
(My TP-Links do not have "Bridge mode with AP", it is possible this mode would also be a good choice)
If you listed the IP address range you want both buildings to have and the router IP address, I could give more detail, but I can't make that up.--
I tried to remain child-like, all I achieved was childish.