Start from Apple OS 14.x or later iPhone cellular DNS modification is difficult, it is no longer can be done manual under Settings > General > About > Cellular Provider > Version. Therefore we suggest to resolve it from 3rd party software providers, namely, 1) DNS Override, 2) 1.1.1.1: faster internet.

For DNS Override
1) Download > DNS Override (https://www.dnsoverride.com/) or (https://itunes.apple.com/us/app/dns-override/id1060830093?mt=8)

Installation procedure is as follows > https://www.dnsoverride.com/change-iphone-dns-with-an-app/

For 1.1.1.1: Faster internet
2) Download > 1.1.1.1 (https://apps.apple.com/hk/app/1-1-1-1-faster-internet/id1423538627)


install it to resolve DNS problem which unlock the DNS limitation from your local ISP network provider.
Then you can carry on with Cowbee SDN-VPN service which subscribed.

Good luck!