[SOLVED] Connecting 2 Computers

Solution
The second router is on the other side of my house and acts as a secondary router I can't connect the first PC directly to that router due to multiple walls and different floors.
Well, your diagram cannot work.
The ISP talks to one device, the ONT.
The ONT talks to one device, the router.

Additionally, the ONT is the actual conversion thing between the signal that exists in the fiber, to a signal that exists in ethernet or coax. You can't simply replace it with "a normal router".
No for a number of reasons which partially be due to your diagram if it does not actually work that way.

First a ONT works as a modem. The ISP will only give you a single IP address so you can connect only 1 device to the ONT. So only your pc1 or the router will get a IP address the other device will not function. This is why we even need a router, its main purpose for a home user is to share this single IP address between machines. That is also one of the reasons that a modem or ont only has a single ethernet port

So lets assume the ONT actually is not just a ONT and has a router. From the viewpoint of pc2 everything past the router including PC1 is on the internet. To some limited extent pc2 can access pc1 but pc1 can not intiate communication with pc2 for the same reason some random device on the internet can not attempt to talk to your computers in your house.

Pretty much the only method that works well is ISP----modem/ONT-----router----devices.
 
No for a number of reasons which partially be due to your diagram if it does not actually work that way.

First a ONT works as a modem. The ISP will only give you a single IP address so you can connect only 1 device to the ONT. So only your pc1 or the router will get a IP address the other device will not function. This is why we even need a router, its main purpose for a home user is to share this single IP address between machines. That is also one of the reasons that a modem or ont only has a single ethernet port

So lets assume the ONT actually is not just a ONT and has a router. From the viewpoint of pc2 everything past the router including PC1 is on the internet. To some limited extent pc2 can access pc1 but pc1 can not intiate communication with pc2 for the same reason some random device on the internet can not attempt to talk to your computers in your house.

Pretty much the only method that works well is ISP----modem/ONT-----router----devices.
Can I change the ONT with a normal router and have it connect in some way?
 
The second router is on the other side of my house and acts as a secondary router I can't connect the first PC directly to that router due to multiple walls and different floors.
Well, your diagram cannot work.
The ISP talks to one device, the ONT.
The ONT talks to one device, the router.

Additionally, the ONT is the actual conversion thing between the signal that exists in the fiber, to a signal that exists in ethernet or coax. You can't simply replace it with "a normal router".
 
Solution
The second router is on the other side of my house and acts as a secondary router I can't connect the first PC directly to that router due to multiple walls and different floors.
However you can move router near ONT and connect into ONT LAN socket. Then connect both first and second PCs to router LAN outputs. Problem solved. Particularly in my home internal network works like that. Plus I have TV top box in another room with a kitchen in middle, connected with Ethernet cable too. Obviously if you want to have Gigabit Ethernet for both PCs and current router support only 100 Mbps Ethernet then you must have another router.

Switching ONT into router mode may be possible. However I strongly believe that your ISP will discourage that.
 
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