If I've read this correctly, your 4G Router is a fully-functional, NAT router that provides Internet access via its LAN ports (ie you can plug a PC or Wifi Access Point into them and get access to the Internet without any additional router).
If that is the case, you should probably link it to one of the 2860's LAN ports, not WAN2. You can use the 2860's VLAN options, to establish that LAN port as being on the 192.168.8.0/24 network (and give the 2860 an additional, static IP address on that network). If you then enable InterLAN-Routing (with or without Firewall Rules), I think it should give you what you want.
As it stands, all traffic outbound from the 2860 LAN, will be double-natted - once by the 2860 and once by the 4G Router. Depending on what you're trying to access, this can be highly undesirable. When it comes to inbound traffic from the 4G Router's LAN, heading for the 2860 (assuming such a Route is defined), the 2860 thinks that traffic is coming from the Internet (and treats it accordingly). It will apply NAT and will only pass the traffic through if there is a matching outbound session, or NAT Open-Ports/Port Mapping rules are in place.
UPDATED
There will need to be a Static Route on the 4G Router, that says 192.168.1.0/24 is accessible via 192.168.8.105 (or whatever you give the 2860) ... and not via the 4G Router Default Gateway (which will be somewhere on your ISP's network).
You shouldn't need a static Route on the 2860, because it will know already where the 192.168.8.0/24 network is - unless the 4G Router is your only route to the Internet. In this case, the 2860 needs that to be specified as its Default Gateway. This might be achieved through either Route Policy or a Static Route definition for 0.0.0.0. (A bit more research may be required here :wink: )
