Never point WAN DNS in your GT-AC5300 WAN configuration to your own local Pi-hole/Unbound DNS servers. Functions such as time zone, etc. will not work within the router, and you'll run into other issues as well. WAN DNS in your router needs to point to something upstream for DNS such as Google (8.8.8.8) or Cloudflare (1.1.1.1, as an example). Clients on your LAN...PC's, smartphones, Wifi outlets...those should all be configured to point to your Pi-hole DNS servers...that way you get all the benefits of the DNS filtering/privacy offered by Pi-hole/Unbound, none of your local clients will use the router for DNS. I'm nearly 100% certain this will resolve your issues.
Pi-hole is the only entity that points to Unbound for upstream DNS, since Unbound binds to the localhost 127.0.0.1 IP running on the Pi-hole system. Nothing can being "pointed" at Unbound directly, your LAN clients must point to the LAN IP address(es) used by your Pi-hole systems (192.168.1.x for example). Pi-hole handles DNS....with a recursive Unbound DNS setup Pi-hole will forward all external DNS requests through Unbound recursively, rather than going upstream to Google or Cloudflare, which is what Pi-hole does by default.
I'm running the latest Pi-hole (with Unbound) on the newest ASUS 42643 GT-AC5300 firmware with zero problems. Make sure you're using upstream public DNS (Google DNS, Coudflare, your preference) for DNS 1 and DNS 2 server settings within the WAN settings (under Advanced in the web UI).
Do not point WAN DNS at your Pi-hole(s).
Only your LAN DNS server (Advanced -> LAN -> DHCP Server) should point to your Pi-Hole server LAN IP. Since you're running multiple Pi-hole/Unbound servers, I'd recommend configuring your LAN clients to manually point to those private IP's for DNS manually, rather than having the router assign via DHCP, since the router firmware only allows 1 DNS server IP to be entered within the LAN settings. Ensure that Pi-hole is set to use Unbound as the upstream DNS...should be 127.0.0.1#5335 for CUSTOM Upstream DNS within Pi-Hole DNS Settings. Also make sure you have disabled Pi-hole for DHCP if you have DHCP enabled in your router, or you'll run into all sorts of problems.
Follow this guide to ensure you configure Pi-hole and Unbound correctly for recursive DNS -
docs.pi-hole.net
In Pi-hole Advanced DNS settings, ensure you have the following boxes
ticked:
*
Never forward non-FQDNs
*
Never forward reverse lookups for private IP ranges
Ensure that "Use conditional Forwarding" is
UNCHECKED.
I also recommend setting up and using Local DNS records within Pi-hole, if you want to assign local DNS names to clients on your local network (makes it easier to connect remotely to PC's, etc. instead of having to remember IP addresses). If you're running two Pi-hole/unbound systems, one of the Pi-hole's should be set up as a "slave" failover DNS server unless you have specific reasons to run separate independent DNS servers in your own home local network. I've found that one Pi-hole DNS server is more than sufficient for home private networks.
I have roughly 55 clients locally on my LAN (wifi outlets, PC's, game consoles, etc.) and none of them have had any issues with DNS. You do have to ensure your configuration is correct between the router (GT-AC5300) and your Pi-hole/Unbound servers. Avoid using .local for your local domain name. I've found local domains such as .lan or .homelan, etc. to work much more reliably. Mac systems have issues with .local since mDNS uses it.