I think this is what you want to do?
ISP ISP
| |
R1 --fibre-- R2
| |
LAN1 LAN2
but R1 and R2 are not (afaict) actually 3-port routers - they only have 2 IPs - one for the WAN and one for the internal eth port connected to the presented 'switch ports'.
I'd buy 2 more super simple routers and do:
ISP ISP
| |
R1 R2
| |
LAN1--R3--fibre--R4--LAN2
in which case you don't need NAT and R3 needs a static route to LAN2 and R4 needs a static route to LAN1
Looking at:
LAN1--R3--fibre--R4--LAN2
is really:
LAN1 -- eth/lanport-R3-wan ---fibre--- wan-R4-eth/lanport -- LAN2
so wan on the routers is basically another network. Many years ago on real pt2pt links I'd set this up as an ip-less connection but you may have to make it a subnet 192.168.3.0 would be fine.
Now you'd need to assign an IP to the R3/4 lan and wan
You'd set
R1 to static route to LAN2 via R3/lan
R2 to static route to LAN1 via R4/lan
R3 to static route to LAN2 via R4/WAN
R4 to static route to LAN1 via R3/WAN
default route on LAN1 would be R1
default route on LAN1 would be R2
Doing the R1->R3 avoids setting static routes on all machines in LAN1/2 which would otherwise be needed and would be a pain.