Next.js + WordPress Website Deployed on Vercel – Inconsistent 404 Errors Across Devices

Question

I'm facing a perplexing issue with my website after deploying it to Vercel. Here's the setup:

• Technologies: Next.js (unfamiliar to me), WordPress (subdomain), Faust, GraphQL plugin
• Vercel Configuration: Faust secret key and WordPress URL set within Next.js project
• Domain: Custom domain added and verified in Vercel.

My Problem:

The website works seamlessly on the Vercel domain. After attaching my custom domain, the website loads inconsistently across devices:

• Works on some Android devices
• 404 errors on iPhone 13 and some desktops
• Inconsistent results from web speed and load testing tools

My Questions:

Faust Frontend Site URL: Should this be set to my Vercel domain or the custom domain? Next.js public_site_url: What should the optimal value be in this scenario?

I've spent three days trying to find a solution without success.

I would greatly appreciate guidance from the Stack Overflow community regarding:

• What could be contributing to these device-specific errors?
• How can I effectively isolate the source of the problem?
• Are there any configuration tips for this specific setup to ensure consistency?