That is the clue
The problem was in the PC
/etc/ssh/sshd_config
had ListenAddress set to my local net.
That disables all other sources… such as my modem’s wifi net.
SOLVED
Have to be careful with username… Termux defaults to u0_a304 which does not exist in the PC
It now has a known_hosts file .
That is not quite the full story. I then changed the PC from Void to MX, and the ssh from Android failed. This time it was because MX had ufw enabled. I had to allow port 22 for it to work .
Then when I move the PC back to Void, it fails… the key for trinity is wrong. I have to delete known_hosts for it to work again.
ssh is hopeless in a multiboot environment.