Depending on how they do their length check, this “English only” check makes sense in a way. Non-ascii letters are a pain to validate the length of (“🏴󠁧󠁢󠁳󠁣󠁴󠁿” is seven code points and fourteen bytes long, but just one “character” to most people).

I don’t know why the max length is that low (I presume it’s stored in plaintext) but you can’t expect someone to write a good password strength algorithm from a company producing the cheapest routers imaginable.