This looks like a Wireguard issue, rather than a Pi-hole one.
You should probably consider to also consult forums specialising in Wireguard.
That said:
From what I can glance from your configs, it seems you tell your phone peer to route all traffic via Wireguard:
However, on your Pi, you did not configure Wireguard to setup any rules that would route traffic from the Wireguard network to an interface with the required connectivity, as you are missing the appropriate PostUp/PostDown rules.
This would match a disclaimer from our Wireguard VPN guide:
The following assumes you have already prepared your Pi-hole for IP forwarding and enabled NAT. If this is not the case, follow the steps over there before continuing here.
If you haven't done this, your clients will apparently have no Internet connection whilst local connections may still work.
I'd recommend to have a read of aforementioned guide, or at least follow above links from the quote.
Please note that for Pi-hole to provide DNS via VPN, it would suffice to use your Pi-hole's Wireguard IP for DNS (in your phone.conf, DNS = 10.100.0.1), while only routing traffic to that DNS server via Wireguard (in your phone.conf, AllowedIPs = 10.100.0.1/32, fd08:4711::1/128).
In that config, only your phone's DNS request to 10.100.0.1 (or fd08:4711::1) would be sent into the VPN tunnel.
All other traffic would be routed through your phone's normal current connection (wifi or mobile).
This would allow your phone to benefit from Pi-hole's filtering while still enjoying the download speeds of your phone's connection (e.g. 150Mbit /s of mobile download speed).
You'd only need those additional PostUp/PostDown rules if you'd wanted to be able to access devices within your home network via VPN (like accessing your router at 192.168.1.1) or route all traffic via your home network.
Note that any traffic that has to be sourced via your VPN would have to be sent via your router's upstream link - and for most consumer plans, upstream speed is considerably lower than download.
Consequently, routing all traffic through a Wireguard tunnel may noticably slow down your phone (e.g. assuming 150 Mbit/s download for the phone's 4G mobile data vs. 10 Mbit/s for your home router's upload speed).
Routing DNS traffic only would be much less likely to be noticeable, as DNS packets are usually quite tiny and make up only a small fraction of overall traffic volume.