"Ethernet" ports can be twisted pair or fiber.

Assuming you mean the router has got TP ports only: there's a wide range of media converters out there, converting 1000BASE-T to e.g. 1000BASE-LX. Many media converters have no fixed fiber port but a modular SFP port that allows you to plug the exact PHY module you need. Make sure it matches the one that's used on the other end.

If the router/switch already has an SFP port you should use a fiber module in that as it avoids the separate hardware and gives you more control over the fiber port.

You can also use a switch as "media converter". Make sure you separate the fiber traffic (WAN?) from your internal LAN by configuring VLANs appropriately. Also, you might want to deactivate STP and such to avoid disturbances of your internal network.

"Ethernet" ports can be twisted pair (TP) or fiber.

Assuming you mean the router has got TP ports only: there's a wide range of media converters out there, converting 1000BASE-T to e.g. 1000BASE-LX (for duplex single-mode fiber). Many media converters have no fixed fiber port but a modular SFP port that allows you to plug the exact PHY module you need. Make sure it matches the one that's used on the other end.

If the router/switch already has an SFP port you should use a fiber module in there as it avoids the separate hardware and gives you more control over the fiber port (improved link-loss signaling, possibly signal-strength monitoring).

You can also use a switch as "media converter". Make sure you separate the fiber traffic (WAN?) from your internal LAN by configuring VLANs appropriately. Also, you might want to deactivate STP and such to avoid disturbances of your internal network.

"Ethernet" ports can be twisted pair or fiber.

Assuming you mean the router has got TP ports only: there's a wide range of media converters out there, converting 1000BASE-T to e.g. 1000BASE-LX. Many media converters have no fixed fiber port but a modular SFP port that allows you to plug the exact PHY module you need.

If the router/switch already has an SFP port you should use a fiber module in that as it avoids the separate hardware and gives you more control over the fiber port.

You can also use a switch as "media converter". Make sure you separate the fiber traffic (WAN?) from your internal LAN by configuring VLANs appropriately. Also, you might want to deactivate STP and such to avoid disturbances of your internal network.

"Ethernet" ports can be twisted pair or fiber.

Assuming you mean the router has got TP ports only: there's a wide range of media converters out there, converting 1000BASE-T to e.g. 1000BASE-LX. Many media converters have no fixed fiber port but a modular SFP port that allows you to plug the exact PHY module you need. Make sure it matches the one that's used on the other end.

If the router/switch already has an SFP port you should use a fiber module in that as it avoids the separate hardware and gives you more control over the fiber port.

You can also use a switch as "media converter". Make sure you separate the fiber traffic (WAN?) from your internal LAN by configuring VLANs appropriately. Also, you might want to deactivate STP and such to avoid disturbances of your internal network.