The meaning is not entirely clear, but I assume "forwarding" would at least mean your router is accepting DNS queries (by distributing its own IP address as local DNS server via DHCP) and then forwards them to your chosen upstream (and that's completely in line with you observing it as Pi-hole's only client, apart from Pi-hole itself).
However, "forwarding" could also mean that your router additionally is catching and redirecting all outbound DNS traffic and forwards it to the configured DNS servers. Depending on how that's done, it may be a good or a bad thing with regards to Pi-hole.
And depending on your configuration decision (see below), it then may or may not be advisable to revert that "forwarding" option to its defaults.
You'd have to consult your router's manual and/or support to confirm what Lancom means by "forwarding" exactly.
It means you have four options:
a) live with the status quo: your router is forwarding DNS requests to Pi-hole, effectively making it your only client.
b) try to locate a router menu option that would allow you to distribute Pi-hole as local DNS server via DHCP (replacing your router) - this option is commonly found in the LAN or DHCP section of your router's configuration UI.
c) try to disable your router's DHCP server and use Pi-hole's DHCP server instead.
Contrary to your assumption, Pi-hole's embedded
dnsmasq is quite capable of serving DHCP. There are even routers out there that use
dnsmasq under the hood as well.
d) manually configure Pi-hole as DNS server on each single device.
Again, as configuration options are specific to a router's make, model and firmware, you'd have to consult your router's documentation on your available options.
It may also help to disclose your exact router model, preferably also in this topic's title, in order to better attract users with the same or a similar router. They might be able to share their experience and to provide more specific advice on its configuration.