Your debug log shows your Pi-hole to have problems with its own IPv6 address:
*** [ DIAGNOSING ]: Networking
[✓] IPv6 address(es) bound to the eth0 interface:
2a01:<redacted>:8a0c does not match the IP found in /etc/pihole/setupVars.conf
fe80::<redacted>:3d34 does not match the IP found in /etc/pihole/setupVars.conf
*** [ DIAGNOSING ]: Name resolution (IPv6) using a random blocked domain and a known ad-serving domain
[✓] impexrubber.co.in is :: via localhost (::1)
[✗] Failed to resolve impexrubber.co.in via Pi-hole (2a01:<redacted>:5f38)
[✓] doubleclick.com is 2a00:1450:4001:821::200e via a remote, public DNS server (2001:4860:4860::8888)
This is because your public IPv6 prefix changed.
Consider to Use IPv6 ULA addresses for Pi-hole.
As iPhones (as almost all smartphones) would prefer IPv6 over IPv4, lack of a correct IPv6 address for Pi-hole may have contributed to your issue.
However, a change of your public IPv6 prefix (2000::/3 range) is triggered by your ISP (or a router restart, eventually) and would affect all of your network, not juts your iPhone.