Range extender for Atheros based Netgear Nighthawk R7800

My Netgear Nighthawk R7800 (atheros) uses Openwrt and I am searching for a range extender. For unknown reasons a Netgear EX6150v2 with Stock-FW (Openwrt planned in the future) has a noticeable stronger signal than an Archer C7 running Openwrt, when checking with an Android app. Difference is about 10dB.

The Netgear EX6150v2 is sold out and the EX7300 doesn't support Wifi 2.4.

Searching the database, it is hard to find a range extender, which can be bought as new.

Any ideas what to buy, which is better than the Archer C7 as range extender?