I have fixed this problem by implementing the fixes described by @PaulS in this post:
AS-750S Slate Rotuer: DNS fix to allow use of internal DNS server with Wireguard Client
In the post he links to a blog wherein he describes the changes in detail:
GL-iNet GL-AR750S-Ext mss clamping mtu_fix for Wireguard VPN
Note: I did not need to implement the mtu_fix, only the two dns fixes.
You can read about how to make this change in @PaulS blog above. Here is a patch for “/etc/init.d/wireguard” file on the AR750:
--- /etc/init.d/wireguard.old 2019-07-11 09:30:43.563687226 +0200
+++ /etc/init.d/wireguard 2019-07-11 09:30:25.301885360 +0200
@@ -95,6 +95,7 @@
#mv /tmp/resolv.conf.auto /tmp/resolv.conf.auto.hold
echo -e "nameserver $dns" > /tmp/resolv.conf.vpn
uci set dhcp.@dnsmasq[0].resolvfile='/tmp/resolv.conf.vpn'
+ uci add_list dhcp.lan.dhcp_option="6,$dns"
uci commit dhcp
/etc/init.d/dnsmasq restart
else
@@ -382,6 +383,7 @@
[ -f "/tmp/resolv.conf.vpn" ] && {
rm -rf /tmp/resolv.conf.vpn
uci set dhcp.@dnsmasq[0].resolvfile='/tmp/resolv.conf.auto'
+ uci del_list dhcp.lan.dhcp_option="6,$dns"
uci commit dhcp
/etc/init.d/dnsmasq restart
}