Let's Encrypt does not have an official support team; this is how we are able to operate for free. The kind folks who reply in this forum are volunteers and community members.
It is not possible to resume issuance from R10 and R11; those intermediates have been retired.
Your systems should not care which intermediate was used to issue your certificate -- this is why we rotated randomly between R10 and R11 for the past year, why we switched from those to R12 and R13 recently, and why we now rotate randomly between those two. You should not rely on a particular intermediate being used.
As suggested above, you should instead rely on specific roots being used. For Let's Encrypt, those roots are (currently) ISRG Root X1 and ISRG Root X2. You can find their details on our certificates page: Chains of Trust - Let's Encrypt