I replaced pre AR-750S devices by AR-750S too. The device have a flash which is big enought for newer firmwares, if available without show stopper bugs like DNS leak. On this time are the last one known firmware for this device (and some other) without DNS leak:
- Firmware ar750s-3.105-0104
For my experience and known status of bug fixing, you can check the follow:
- Firmware openwrt-ar750s-3.201-0402.tar, possible still same DNS leak or again
- 2021, gl-inet Firmware 3.2, shorttest of snapshot version
For alternate and possible DNS leak free Firmware, you can check the follow: