Sorry to revive this thread, but I have searched and it is the most recent one with the problem. I just updated (reinstall, no keep-config!) from LEDE 17 to latest stable 18.06.4 and have the same problem. This is NOT about the default shell, ASH, but the packaged and available to install BASH. It worked before LEDE, as I had openwrt designated driver working good before, too.
The completion of commands and filenames works, ONLY if there is only a single match (i.e. you type tcpdu and it completes to tcpdump). The ALT+= combo works too.
It is not a xterm thing as I have tried to login from the TTY too (TERM=linux).
Has to be something that changed with the 18.06 bash package compile flags or terminal settings, or a default package that was installed before as a dependency and now it's not.
Edit: Google fu "bash ambiguous" found it elsewhere! Has something to do with readline defaults. Putting it here so anybody else can benefit.
bind "set show-all-if-ambiguous on"