Changes made to /etc/resolv.conf get reverted when the server reboots.
Most cloud hosting providers install packages to manage the resolver configuration on their servers. These packages reconfigure the resolvers at boot.
There are several packages that cloud hosting providers can install to manage the server's resolvers. The best option to prevent modifications to the resolvers from reverting at boot is to work with your hosting provider to adjust the configuration of the package that manages the resolvers, so that the modified resolvers are set at boot.
Alternatively, the immutable flag could be set on /etc/resolv.conf to prevent modification of the file.