MRNA vaccines are not banned to sell in poorer countries. They are currently just too expensive for them, I suppose.
Once Moderna recoups the cost of R&D and pockets some profits, it will face the need to keep selling. With the pandemic basically over, they'll need to sell to wider markets, because they have the production capacity. They'll lower the price and try to use the economies of scale by selling large amounts.
The fact that any technological advances first become available to those who can pay a high initial price (not only in money) is pretty inevitable. If something is a runaway success, the price goes down with volume, and ideally with pressure from competition.
If you want to make something vitally important available at a low price, you have to find enough money to let the producers make some profit, or at least to recover the cost of development. If you don't, nobody will consider working on it, even if they wanted, because getting an investment would become impossible, and you can't do biotech on pocket money.
(You could of course force them work at gunpoint, but not being forced to work at gunpoint is usually even more vitally important for the health of a society.)