My router only supported Merlin (Tomato) firmware up to 380.70_0 (2018-04-08) and there are no more updated from that branch.

Turns out the Namecheap DDNS client is not working. I searched its source code at Github for “Namecheap” to see if I can fix the HTTP 400 error (seen in router log) using “in:file” hoping to see if I can fix the bug myself (since it’s just a simple REST API, aka URL call) command and found this:

It means Asuswrt-Merlin (Tomato)’s firmware’s Namecheap DDNS service is broken UNTIL 384.7_2, which is for newer routers than what I have.

I compared the “namecheap” keyword search for the two versions and it seems like they are written very differently, so it might not be worth the effort to fix the obsolete Merlin branch. Doing a bit of additional search and discussions about John’s Merlin fork from the early days are still updated until 2019-04-17 :

It took me a while to find John’s forum post to get to the said 2019-04-18 release. The download link he provided has more updates up to Update-44EA (2020-07-12) and it still worked on my old router, so it’s not just the Update-39L3 previously discussed in the forum:

Do not get discouraged when John’s Merlin fork says version 374.43 instead of 380 (a lower number). All updates, even the latest one from John still use 374.43, which I suppose it’s to mark when the two code branches part ways. It’s not an older version.