You should be able to book reservations free of charge on GWR. They're optional though:

https://www.seat61.com/interrail-and-eurail-reservations.htm#UK

If that doesn't work for you, then please mention date and departure time.

The date would by Monday 17 June. Is it to far out to book?

That's quite possible. Check the same train for an earlier date, like in 2 weeks. If it shows up as bookable, then you just need to wait.

I´ve booked my seat last year by conntacting Transport for Wales TfW as they operate that train. 

The issue with this route is that they use often replacement trains. I especially wanted to use the so called Premier Service with Dinning offer in 1st class. But quite often this train is replaced by a small unit without any food facility (with luck a trolley service) 

 Wikipedia Page about Premier Service 

 

TfW currently do not offer seat reservations on any of their trains, even the premier service.

https://tfw.wales/ways-to-travel/rail/travel-information/reserve-your-place

Just turn up and board, Holyhead is typically not that busy so normally you won’t have any problem getting a seat there, the line towards Chester is very well used and you can expect the trains to fill up along the journey, especially when short 2-car units are used.

It typically takes about 25-30 minutes from ferry arrival in Holyhead to getting to the train station so, you won’t catch a train scheduled to depart any sooner than that.

The food on the premier services is excellent, right up there with the best you can get on any scheduled train in Europe, the issue with the full service trains being replaced with units not equipped with catering is ongoing, the best chance is going for the 16.49 Manchester or 17.14 Holyhead from Cardiff,  it’s unusual for neither of these to run as booked.