Force SQL Server to use fragmented indexes?
Index Hint

You can use an index hint to do that:

SELECT *
FROM dbo.Users AS u WITH (INDEX = ix_definitely_an_index)
WHERE u.Reputation = 2;


The downsides are:


Potentially changing a lot of code
If you rename an index, this breaks
If you change an index definition, it might not be the best index to use anymore


Plan Guides

You can also use a Plan Guide, which sets the plan you want in place.

The downsides are:


It's fickle; if your query isn't exactly the same, your plan might not get used
It's tough to set up; you have to get everything exactly right
Also the same stuff as an index hint, so uh... That's rough.


Query Store?

Since you're on SQL Server 2017, you may be able to use Query Store to force plans. This is much easier than using a Plan Guide, but you may end up with an unexpected plan.

Downsides:


Same stuff as above
Having to enable Query Store, if you're not already using it
Having to query Query Store to see if forced plans are failing


Alternatives

Without knowing more, I'd probably be more keen on investigating if the index use changes for other reasons, like parameter sniffing.