as a guy who discovered another xiaomi device a few days ago (miwifi mesh), i can say that probably no telnetd is available inside the stock firmware
telnetd busybox applet for my device never run, no matter which value telnet_en 0 or 1, it just start and exit, probably telnet code replaced with dummy {} to prevent any kind of enabling it.
also no dropbear included in "stable" firmware usually, so uart is a single way to access device from inside, but as transmit to router disabled by default, you need to force it into tftp recovery mode using reset buttons and interrupt first boot at u-boot, enable uart_en, boot to stock firmware and modify any startup script to set uart_en to 1 at every boot (for example you can use /etc/rc.local)
to get a working telnetd, you need standalone static busybox binary with telnetd compiled for your target arch