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) 188.8.131.52: 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)
For 184.108.40.206: Faster internet
Installation procedure is as follows > https://www.dnsoverride.com/change-iphone-dns-with-an-app/
2) Download > 220.127.116.11 (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.