local DNS across subnets ? - how can I make FG share all known .local adresses ?

My internal network 192.168.1.x, with FG being a DHCP/DNS server.. it has a Linux device registering as pi4.local  - all fine  - I can ping/resolve pi4.local

Then there is a PoE lan, with 192.168.2.x - this too  provides DHCP/DNS,  - except if a pi3.local registers there, then I can lot ping pi3.local from the internal net.  (rules allow full access)

The problem is that local DNS lookup for "internal" does not include devices registered in the other network.

How can I make FG's DNS provide all *.local adresses  - across networks?