Facebook loaded slowly (could not be loaded) and some webpages could not be accessed when setting dns 1.1.1.1 for router from an ISP in Vietnam

Dear Experts,
Recent times, facebook loaded very slowly (sometimes could not be loaded), and some web-pages I could not access (before it still ok), both on my mobile and windows. I removed 1.1.1.1 that was set for dns in router (huawei) or changed it to google dns, it works. My isp is Viettel in Vietnam (Ho Chi Minh city), the web browser is chrome, are there any problems with this isp or does your service support Vietnam or this ISP yet?
I also tested by keeping 1.1.1.1 for router, but dns in windows set google dns, it still works.
Please help me to identify the problem.
Thank you so much.
Kind regards,
Don
Ho Chi MInh city, Vietnam

Yes, I experience this issue too, and it is by design.

Cloudflare doesn’t send our IP address to “authoritative servers/nameservers”, so sometimes we get further servers (no geo-DNS etc)

Usually, if I use 1.1.1.1, I get Facebook Singapore IP instead of my country. But if I use google DNS, I would get the closest location of facebook IP.

This is by design, nothing you can do about it. https://developers.cloudflare.com/1.1.1.1/faq#does-1111-send-edns-client-subnet-header