For a Mathematics scholar, who is interested in Cryptography, AI and Computer Science related fields, are there any post-doc researches available to be done by online/distance learning mode?
If there's a way to do such a one please kindly advise
You are in luck because most work in those fields does not need specialised equipment and can be done remotely.
Whether there exist positions that allow you to do it vary on some factors:
Overall, the answer depends on how remote you need the position to be, but it is possible that some positions exist that satisfy your criteria. Note, however, that no position will be marked as such: you need to reach out to the potential supervisors and ask whether they would be okay with a remote postdoc.