Hello,
I inherited a situation where I have an ancient phone line connected to an ancient Netgear DSL modem/router.
If possible, I would like to place a Mikrotik router as shown in the picture so that it can see the two private LANs.
The 10.0.0.0 LAN has no hosts except for the Netgear modem/router. The 192.168.1.0 LAN has several computers in it.
Could the Mikrotik router act as some sort of forwarding bridge between the 10.0.0.0 LAN and the 192.168.1.0 LAN.
How could I set this up?
Many thanks in advance.

Could the Mikrotik router act as some sort of forwarding bridge between the 10.0.0.0 LAN and the 192.168.1.0 LAN.

Yes, but it is not a "forwarding bridge" it is a normal router setup.
Any request in the network for addresses outside 192.168.1.0 will go to the Mikrotik gateway 192.168.1.1 that will then route them to "next hop", the Netgear gateway at 10.0.0.1.
You might even restrict the connection between the Netgear and the Mikrotik to just two addresses, 10.0.0.1 and 10.0.0.2 by using a network of 10.0.0.0/30, if you have access to the Netgear router settings I would disable the Netgear DHCP and use static addresses on both the Netgear and the Mikrotik.
It is essentially the simple routing example here:
https://wiki.mikrotik.com/wiki/Manual:S ... ic_Routing
where router1 is the Netgear, router2 is the Mikrotik and you have only LAN2.

You could achieve this by multiple ways.

1. Static route on the Netgear for the 192.168.1.0/24 network
2. Convert the mikrotik to a switch(bridge all ports) and use the Netgear for the network(the mkt could still firewall)
3. Do a second nat on the mkt

With the current setup ( assuming the MT is setup as a router ) and ensuring the masquerade rule on the Mikrotik router, any traffic heading out of the mikrotik is going to go to two locations.
a. out the internet - one of your users wants to browse, email etc.
b. or to a local subnet behind the ancient DSL - one of your users wants to visit a LAN device on the ASDL Lan. ( in this case a single subnet I believe )

The return traffic will reach the originator because the all local MT traffic is masqueraded to appear as the LANIP of the MT router on the ASDL LAN and thus the older router will send all such traffic back to the MT which will then unNAT the traffic and send it to the correct originator.

What you cannot accomplish here is have someone on the ADSL LAN reach someone on the MT LAN (easily).
The reason is that the ADSL router knows nothing about the MT LAN and thus will not know where to send such traffic.

Two possibilities that will work without much effort.
a. IF you want to reach a LAN server./device, then think of this as a port fowarding issue. Lets say one has a server at 192.168.1.15 and its working port is 12000
Then any user on the ASDL can reach this server simply by using 10.0.0.2:12000 as the entry argument.
The ADSL recognizes the local IP and sends traffic to the MT router......
One just needs to setup the MT router to port forward that request to the server at 192.168.1.15

b. If you want to reach the router for config purposes. One can do this from the ADSL LAn.
Simply use 10.0.0.2:WINBOXPORT as the entry argument.
One needs to setup on the MT router an input chain rule allowing that ADSL LANIP access.

In Summary:
There is no issue in the current setup of MT LAN users reaching the ADSL LAN.
In the current setup, with some minimal MT configuration, ADSL LAN users can reach LAN devices or the MT router itself.

Does this satisfy your requirements?