I will add to others’ comments, that although this is the default on the communities list, make sure the “Local” option is selected to see all communities on the specific instance or “All” to see all communities the instance federates with (in other words, all the communities it allows access to)

Assuming we can use both lower- and uppercase letters (52 in total), with the ten digits and the underscore that gives us 63 characters to work with. A random 16-character combination of these gives us 95 bits of entropy (rounding down), which is secure enough by modern standards, at least for a home router.

Regardless, I understand the frustration of arbitrary limitations preventing you from choosing a secure password in a way that you’re comfortable with.