Thanks for the info, I am not seeing any issues with the site here.
A 521 error happens when Cloudflare is unable to make a TCP connection to your origin server. Review the suggestions in this Community Tip for Quick Fix Ideas.
I would suggest, given that the issue appears to be location specific, that your host may be rate limiting or blocking some Cloudflare IPs which is what may be affecting you. Are you able to ask them to whitelist the Cloudflare IPs?