https://roderikdeblock.com/vmware-uag-not-using-dns/

Edit /etc/systemd/resolved.conf and uncomment the domains line and adding in your .local domain to the domains there.

---

Follow the docs for your version like

https://docs.vmware.com/en/Unified-Access-Gateway/3.2.1/com.vmware.uag-321-deploy-config.doc/GUID-53...

look for

Note:If you are not able to access the Admin UI login screen, check to see if the virtual machine has the IP address displayed during the installation of the OVA. If the IP address is not configured, use the VAMI command mentioned in the UI to reconfigure the NICs. Run the command as "cd /opt/vmware/share/vami" then the command "./vami_config_net".

I think its the same on all versions. Use this to set the dns instead of messing with config files first, usually I've seen dns be a problem because of syntax issues when entering it in the ovf form. 

This might answer a few of your findings:

https://kb.vmware.com/s/article/50120424

Specifically:

Note : Industry recommendations and only use .local for multicast DNS as stated in the RFC. .local is defined for a specific multicast use and not for general DNS. It is covered by https://tools.ietf.org/html/rfc6762 & MS recommendations https://docs.microsoft.com/en-us/windows-server/identity/ad-ds/plan/selecting-the-forest-root-domain (see caution section when selecting suffix). Cnames can be employed to allow alternate names if you have a .local server that you need to alias.

And:

As the UAG is an appliance based on photon o/s, never edit resolve.conf or the host file manually.
Please update dns entries with redeployment or in the admin user interface - note any manual changes to the file will get over-ridden by admin user interface entries or by a reboot.