Ensure the correct setup for your CNAME record
The first thing you need to do is to ensure that your CNAME record has been added correctly inside the DNS settings of your domain registrar.
For example, if you would like to publish a page to campaign.mydomain.com, you would have to create the CNAME record within the DNS settings of your registrar and add it as 'campaign'.
Also, if your registrar is Cloudflare, make sure you bypass their proxy as shown here: https://d.pr/SVJvit.
The information in the DNS management page will, in this case, look like the following:
Record type: CNAME
Host: campaign (Note that campaign is just an example here. The host should be the subdomain you want to publish on)
Points to/value: secure.pageserve.co
For more information on creating a CNAME record, please refer to this complete setup article: https://d.pr/natqMD
Verify your CNAME record with a third-party propagation tool
You would also want to ensure that you waited at least 48 hours for the newly created CNAME record to fully propagate. Once it's fully propagated, the CNAME record will show almost all green ticks when verifying your CNAME record with a third-party web-based DNS Propagation Tool.
To do so:
Visit www.whatsmydns.net;
Enter your custom domain URL in the Domain Name field;
Select CNAME from the drop-down menu;
Click on Search.
The results will indicate whether the CNAME is propagated across 21 servers located worldwide. Check that you see secure.pageserve.co in the right-hand column and that all ticks are green.
If all the ticks are red, this means that the CNAME is not propagating.
If your location/country is not represented in the list of the above-mentioned tool, don't worry. Google Cloud Servers are located all over the world, and your visitors will load the page from the server nearest to them. They even have fail-safe mechanisms to load the page from a different server in the same region in case the server nearest to you has some issues.
If your page has been published for a long time, but suddenly stopped working, first check the CNAME setup according to the instructions above. If the CNAME setup is correct, unpublishing and republishing the page may help.