Problem connecting from a Debian 11 Bullseye to an old OpenWRT Chaos Calmer:

ssh root@192.168.31.45
Unable to negotiate with 192.168.31.45 port 22:
    no matching key exchange method found.
    Their offer: diffie-hellman-group14-sha1,diffie-hellman-group1-sha1,kexguess2@matt.ucc.asn.au

You can edit your $HOME/.ssh/config file and put ah Host stanza like this:

Host hostname hostname.domain.tld 192.168.31.45
    KexAlgorithms +diffie-hellman-group1-sha1

This will force the use of diffie-hellman-group1-sha1 key exchange method, which is considered less secure nowday.