I got a Netgear R7800 on which I tried DD-Wrt.I tried to replace it with OpenWRT but I can't do it. Is there a quick way, perhaps by connecting via Telnet? Thank you.
The usual advice in such cases is to go back to OEM firmware, and from there to OpenWrt.
Is there a method to install oem via Telnet?
The easiest way with R7800 is to use the Netgear's recovery TFTP flash routine (in u-boot bootloader) to flash the OpenWrt factory.img file. The routine can be triggered with the reset button/hole at boot.
The Netgear OEM firmware can also be installed with the same process (and they also give that advice on their site). https://kb.netgear.com/000059633/How-do-I-upload-firmware-to-my-NETGEAR-router-using-a-TFTP-client-on-Microsoft-Windows?language=en_US