Check your DNS Settings. If that does not help, post your configuration, rules and everything that can help us to understand your environment.
Check that you have correctly entered a static route for your LAN on the Fritzbox: https://en.avm.de/service/fritzbox/fritzbox-7490/knowledge-base/publication/show/581_Configuring-a-static-IP-route-in-the-FRITZ-Box/Bart...
1.The route ffritzbox->opnsense-LAN is only needed if you want to access the opnsense-LAN from fritzbox-Net.2.The route is wrong, gateway should be 192.168.2.52 (OPNsense-WAN-IP)3.Don't add any routes in OPNsense, those are added automatially. 4.Can you ping 192.168.2.1 from LAN-Interface in OPNsense?5.Are there any restrictions on the fritzbox? child-protection, online-limit, ...
1) You don't have to, however you can remove the route if you don't need it.5) I'm not sure if i understand.With BIND and/or unbound you can set DNS Query Forwarding. https://wiki.opnsense.org/manual/how-tos/bind.html#advancedYou can put 192.168.2.1@53 there.When adding the IPs of your ISP-DNS-Servers, these mightwill change sometime and break thinks. Be aware of it. Should be fine until then.6.If IPv6 doesn't work: there is a 'feature' in the recent FritzOS. All devices connected to the fritzbox need to be set to standard-profile, without any restrictions.
forward-zone:name: "."forward-addr: IP@53
I'm still not sure which DNS/DHCP you're talking about.DNS-Forwarder @ Services: Unbound DNS: GeneralQuoteforward-zone:name: "."forward-addr: IP@53DNS-server for OPNsense/its DHCP:@ System: Settings: GeneralDNS servers... 192.168.2.1[ x ] Allow DNS server list to be overridden by DHCP/PPP on WANDNS-Server set at OPNsense-DHCP:@ Services: DHCPv4: [LAN]... 192.168.2.1 can be used for testing, "Leave blank to use the system default DNS servers: This interface IP address if a DNS service is enabled or the configured global DNS servers."Above ones "should" set the correct DNS-server at the clients.@unbound the play-button needs to have green background, otherwise its switched off.