The fiber optic cable comes into my house to a telephone box
And that "box" is no doubt an ONT, or it contains an ONT. There is one in every fibre network that I've ever seen.
Frankly, Linksys is a more versatile and compatible product for fibre type connections. Your best bet is to keep using the Linksys router, then connect the Time Capsule to the Linksys router using a wired Ethernet cable connection.
If you are willing to experiment, and have plenty of patience, we could try to get the Time Capsule working with it connected directly to the wall jack.....but there are no guarantees.
The first thing that we would need to know is what kind of connection protocols or credentials your provider might have as requirements in order to allow a router to connect. You may need to contact the support folks at your provider to get that information.
We still do not know who your provider might be. That information is important, because another user reading this post who might have the same provider as you may already know if it is possible to connect an Apple router directly to the fibre connection....or whether it's not possible......since the Time Capsule was originally designed to be used with DSL and cable service.
This type of information would no doubt save you a good deal of time and frustration.