You should tell to your PC/PLC to which IP they should return the packages, without gateway, they will lost the packages due they don't know to which IP should send.
Are right this two rules on RUTX10?
Seems they're right, but I can't be sure of your topology and the connections you already have in your network. My suggestion is to add devices and test them one by one until you get your goal.
- Why did you disable DHCP on second router?
- Why did you set up PC/PLC without gateway?
At this moment i'm a bit lost due the changes you mentioned above. What steps did you get successfully and how can we help you from now?