Then you are missing one of the US locations and the Asian.
See: Public IP addresses of Issuance process - #4 by mcpherrinm
Pay particular attention to the notice that these locations may change at any time and may change without any notice. LE has talked about adding more locations and possibly adding different service providers. There is no automated way to know the LE validation centers.
There are other options than having to be aware of and dependent on specific LE infrastructure.
This wiki explains multi-perspective validation. The section I linked gives other methods and suggestions to give you more reliability: Multi-Perspective Validation & Geoblocking FAQ One option is to switch to using a DNS Challenge. Then LE only needs access to your DNS servers which are commonly accessible world-wide anyway.