The reason Namecheap isn’t working is that’s an IPv6 config, and Namecheap’s DDNS is IPv4-only. As noted here on their site.
Same for DNS-o-matic, it also does not support AAAA records.
There are a lot more DDNS providers that support IPv6 than there used to be, but it’s still far from universal. We’re going to change the DDNS UI so it only shows the IPv6 toggle for services which support IPv6.
Of the free DDNS options, here are a few (in English, excluding several primarily in other languages) which support IPv6:
dns.he.net (though requires pointing NS records for an entire domain or subdomain there)
A number of the paid options, or free with domain registration, also support IPv6. Like I have AAAA DDNS with Gandi on a domain registered there.