This is frequently asked, so searching will find more extensive answers. The bottom line is that pressing the reset button runs a script in /etc/rc.button, which you can edit or remove. Unlike DD-WRT, the whole filesystem is writeable, so it is not a "development" issue to change the script, it can be done after installation as a "usage" scenario.
Telnet has been removed from OpenWrt because it is not secure. SSH can be used to log into the router from either the LAN or the WAN if configured to allow it. A SSH user has root access thus can observe or modify anything.
@aurelionet I do basically everything via the command line and manage my configuration with git. Under the covers, OpenWRT is a reasonably well-featured Linux-based system. The use of UCI is challenging at some times, if you're used to other file-based configuration systems, but most everything is in /etc/config, or in /etc itself.
Edit: I usually directly edit the files. The UCI command or the LuCI GUI have a tendency to "rewrite" the files, stripping out all the comments that I use to remind me what I was doing when I come back a year later