If you're connected to the ExpressVPN, the DNS should be set automagically via DHCP from ExpressVPN.
That's the whole point of using their service. Also, DNS ip addrs can and do change over time. I don't understand why you're manually configuring DNS on your router & pc. Also, a quick Google search turned up a very interesting article about DNS over HTTPS and DNS over TLS support in Firefox, Opera & Chrome. FFox & Opera use Cloudflare's DNS. I only perused the article, although it generally seems to be a manual override, FFOX is actively experimenting and there are different modes. As Cloudflare is or was one of the largest CDNs in the world it would make sense for these browsers to partner with them. Anyway, here's a link to the rabbit hole, should you choose to explore it. https://blog.cloudflare.com/dns-encryption-explained/
