I was able to use the FFD6 command to update memory on Ultralight C, although it seems that's only possible to update the memory 15< page using this command.
To write to >15 page it seems is needed to use a different command something similar to FFA2 I believe.
What is an example of the command?
Without knowing what sw1/sw2 responses you're seeing it's hard to say why you're having that issue. Looking at the datasheet for the ultralight c, there's nothing 'special' about those memory spaces above 0x0F.
You can decode the responses to figure out what's happening with this: https://www.eftlab.com/knowledge-base/complete-list-of-apdu-responses/
There is another APDU you can try though. From the ISO7816 section 4 spec:https://cardwerk.com/smart-card-standard-iso7816-4-section-6-basic-interindustry-commands/
WRITE BINARY: 0x FF D0
Depending on your reader, there might be other commands which allow low level. For instance, some Omnikey readers also have: 0x FF 30
Full command description on page 72: https://www.hidglobal.com/sites/default/files/5321-903_b.5_-_omnikey_contactless_scr_developer_guide.pdf