I have 2 Draytek routers: 2860n and 2862n
Each with own internet, own subnet: 192.168.3.0
Different PCs connected to each routers own subnet

Routers Ethernet connected via LAN ports 2860 LAN3 to 2862 LAN4
Routing table show 192.168.3.0 directly connected LAN3
Routing table show 192.168.4.0 directly connected LAN4
on both routers.

With Inter-LAN correctly ticked on each router
I have added LAN to LAN firewall - Pass Any

Both LANs work fine but independently.
But I still can't link or even ping between LAN3 and LAN2
Any suggestions ?

I mean I can't ping LAN3 to LAN3
from one router to the other.

I'm a little confused :?

you wrote:
Routing table show 192.168.4.0 directly connected LAN4

What's that network and how is it related to the problem? (You said "Each with own internet, own subnet: 192.168.3.0")

(I suspect it might be very important and render the rest of this 'answer' redundant :? )

You also wrote:
With Inter-LAN correctly ticked on each router

If there's only a single IP network involved (192.168.3.0), then "Inter-LAN Routing" isn't involved...

With just a 192.168.3.0 network on each device and a wire between their LAN ports, all the LAN clients should be able to see/hear one another. (Though you wouldn't want a DHCP server on each device, both thinking they are in charge of the same LAN. (But with only a single DHCP server, conveying which Default Gateway to use will be an issue.)

At the moment, are any of the 192.168.3.x addresses duplicated between Vigors? If so the 'Switch' portion (working at MAC level) will just use the first one it 'hears' - it might not be the one you're expecting

Or, is one of these Vigors actually using 192.168.4.0 and the other 192.168.3.0 ?

Sorry, I missed some numbers
Sorry, I don't know how to attach a picture.

Yes correct: one of these Vigors actually using 192.168.4.0 and the other 192.168.3.0

The Arp cache on 2860 only has 192.168.3.x addresses - all on LAN1 Port 1 of router
The Arp cache on 2862 only has 192.168.4.x addresses - all on LAN1 Port 1 of router

I have connected vigor 2860 Port 4 via single ethernet cable and I assigned it IP 192.168.4.253
This connects to vigor 2862 Port 3 and I assigned it IP 192.168.3.253
Routing table on both vigors look correct.

Mouserunner wrote:
Sorry, I missed some numbers

Ah ...

Sorry, I don't know how to attach a picture.

Yup, it's a pain. You have to store them elsewhere (like https://postimages.org/ or https://imgbb.com/) and link to them. Both those services can generate the BBCode for the link.

Yes correct: one of these Vigors actually using 192.168.4.0 and the other 192.168.3.0
etc, etc

What you have configured so far, looks correct to me, for what you're trying to achieve.

Have you configured the "LAN >> VLAN" settings, and assigned the ports to the correct LANs?

Could it simply be a dodgy ethernet cable (or even Vigor Port)? (Are the port link lights on?)

Routing table on both vigors look correct.

Have you defined a static route on each vigor? i.e 192.168.4.0/24 ==> 192.168.3.253 (and 192.168.3.0/24 ==> 192.168.4.253 at the other end)
Can you post the Routing Table from each Vigor

What does Traceroute/Tracert show? ... from Vigor to Vigor, Vigor to remote 'client' and Remote 'Client' to Local 'Client'

Thanks, I'll try sort a couple of pics soon.

Tracert just goes to its own local router.

Port lights are on
but it occurs to me that the arp caches does not include either 192.168.3.253 or 192.168.4.253
Surely every vigor port connected should show its own MAC

I keep thinking this is a spanning tree issue, but can see other no link between LANs