From ONT to router runs ethernet (LAN), so there is absolutely no need to be close each other.
Ethernet is defined to run upto 100m, more than enough inside any home.
Here (in Austria) we get a fiber-connect box where the outside and inside fiber (much more flexibel and robust cable which may use any (even mains) pipe with enough space to pull fiber through) are spliced mostly near inlet where it comes into your home.
Outside fiber is blown through a micro-tube with 7mm diameter.
Inside fiber cable may run anywhere.
So you may put the ONT near this splice-box if you got mains (plug socket) over there and conntect via LAN to router.
Otherwise you use inhouse fiber cable to run near your router and ONT is installed over there.
Often the ONT is located inside a small network closet where all LANs from home comes together. Router may be inside too or on a better position to use its WLAN as well (then you need to run a second LAN back to an optional switch in the closet).
Others prefer ONT near router in living room (fiber and only one LAN between closet and router/ONT).
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