Just in case it helps anyone else, tio (https://github.com/tio/tio) is a great featureful alternative to picocom.
The 841N works with OpenWRT. But AFAIR pretty much an EOL device with not much flash/ram.
Clearly the problem is the firmware was not written in Rust. I suggest a Rust rewrite so that these sort of errors will go away.
I had one of those sitting in a drawer for years. I think i did get OpenWRT on it at one point. Eventually it just stopped working altogether, but i felt like i got more than my money's worth out of it. Even brand new that router cost next to nothing.
Funny enough i have the same multimeter too