Yandex cannot index site dut to error 403

It may not be, but it can help offer an explanation.

InfinityFree uses this security system:

Normally, web crawling bots from search engines such as Yandex are whitelisted, however if Yandex keeps changing the IP ranges they are using, the system, unaware of those changes, would enforce the security checks on Yandex’s requests and cause this issue, much like how Cloudflare can block legitimate Yandex requests thinking they are malicious when this happens.

This is just speculation though; I also do not know how Yandex operates, or how the whitelist is implemented, so I cannot be sure this is the case.