This post is a mix (mess) between several topics related indirectly.
The device ZTE H108n [https://openwrt.org/toh/zte/zxhnh108n ] widely spread by Telecom in Argentina. The only way to flash this router is following @djalfredosolis's instructions on post #4 https://forum.archive.openwrt.org/viewtopic.php?id=42889
The main goal is to make a WiFi extender with this xDSL Modem. I am ok loosing USB and DSL port.
After flashing with 15.05.1, I tried to follow instructions to have Relayd: https://openwrt.org/docs/guide-user/network/wifi/relay_configuration
Unfortunately, package luci-proto-relay is not available for 15.05.1 and this guide is not clear for me on how to implement relayd using CLI.
I installed OpenWRT 19.07.01 (according to https://openwrt.org/toh/zte/zxhnh108n) and I observed that Wireless interface is missing. There is a detailed explanation about this issue published on OpenWrt 18.06.1 Don't show wifi wireless network and don't work . There, it is suggested to install wireless packages manually. I haven't tried that.
My approach was to install manually the packages
luci-proto-relay . It seems it works, since I was able to complete the mentioned Relay-Bridge guide.