Router like this are very low spec for example Fritz!Box 3270 has 360 MHz 1 core CPU, 64 Mb of RAM and 16 Mb of Flash. Only thing you can do with them is flashing OpenWrt, but its latest version doesn’t support devices with or less than 8 Mb of flash and 64 Mb of RAM.
Yes, e2e encryption in Telegram only works in secret chats.