Why my ERT-805 is connected, back-to-back with CB-ERTV35-MT and CB-ERTV35-FC, but the console interface always shows "Protocol Down" and no link to each other at all?

In regard to the protocol down, please add the command "clock rate 64000" to the configuration of serial port. (64000 here is an example connection rate 64k between the two routers)

That is, for example, if you have the router as the following connection.
ERT-805 -- V.35 DCE FC cable -- V.35 DTE MT cable -- ERT-805.

The first ERT-805 use DCE cable will need to have this command.
So, you will need to have the command as like below:
ERT805#config
ERT805#int s0/0
ERT805#clockrate 64000

Then the first ERT-805 (DCE side) will generate the clock with the other device that use DTE interface.

After the command is executed, you should find that the console screen should prompt "Serial0/0 protocol up"

To double check the connection, you can use the command "show int s0/0".

In the last line of the show command, you should find the information like V.35 DTE or V.35 DCE as like the image below.

Finally, if the two ERT-805 should starts to communicate to each physically and logically.