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.