I've tried to read documentation and watch youtube videos about the Asus routers - the AC68U and RT-N12+ in this case, and try to understand the "Repeater mode" which they both seem to support (but the docs for the 68U doesn't include anything about it - seems to be supported only in the latest firmware). Here is a screenshot of what it claims to offer:

Turn a router into a wireless repeater and boost your home Take a look at the DD-WRT client bridge instructions and repeater bridge instructions for additional details and tips/gotchas/problems. You can check if your new setup is working by going to TSV WiFi Range Extender, 750Mbps WiFi Repeater Wireless Multiple Operation Modes: Wireless Router Mode, Repeater Mode, Access Point Mode, and Client Mode (TV/Game Console). Support 2.4/5GHz WLAN networks. With 180 degree Rotating Dual Antennas, faster and more stable. Maximum WLAN security with WEP, WPA/WPA2, WPA-PSK/WPA2-PSK. Downward complies with 802.11g (54Mbit/s) and 802.11b standard (11Mbit/s). Mode Repeater ทั้ง 3 Mode ต่างกันไงครับ - Pantip Universal Repeat Mode, WISP Router Mode, WDS Mode ตัวไหนใช้กับอุปกรณ์ได้ทุกตัว หรือแบบไหนต้องเป็นอุปกรณ์เดียวกัน หรือต้องแลก MAC กันครับ ขอบคุณสำหรับคำแนะนำนะคร

Jun 29, 2015 Bridge Mode and Repeater Mode | Wilders Security Forums Oct 14, 2013 Does the “repeater mode” of the Asus routers support

Oct 14, 2013

Dec 26, 2018 What is difference between `Wireless Universal Repeater The short version: WDS is real bridging, Universal Repeater is a form of NAT where the repeater impersonates its clients. WDS must be configured at both ends. Universal Repeater will work even if it only has a normal client/station connection to the access point it's repeating. – … Repeater vs WISp repeater - D-Link Jun 29, 2015 Bridge Mode and Repeater Mode | Wilders Security Forums