DNS Issue with Web App

Hello! I am working on an issue with a Web App apparently experiencing name resolution issues, possibly following an on-prem network outage. I am attempting to access an on-prem service over our site-to-site VPN connection, which works with an IP but not a name.

The environment configuration looks like this:

• Classic VNET with site-to-site VPN Gateway configured
• DNS servers are on-prem and configured at the VNET
• Under the App Service Plan, the DNS server IP addresses show up both under 'site-to-site address space' and 'ip addresses routed to VNET'
• Web App environment includes WEBSITE_DNS_SERVER variable with DNS server from VNET
• I've re-synced the network at the app service plan level

What works:

tcpping <ip address>:port

nameresolver <on-prem name> <external dns server>

Name resolution for a VM on the same VNET using the on-prem DNS servers

What does not work:
tcpping <on-prem name>:port
nameresolver <on-prem name> #[VNET DNS servers used]

In summary:

• I can access resources over the site-to-site VPN connection from the Web App
• I can access the DNS servers from VMs on the VNET
• I can't resolve names on the web app or Kudu in the ASP

The issue appears to me to be between the app service plan and the VNET. Beyond resyncing the VNET at the ASP, are there other steps I can take to troubleshoot? 

Thanks!