Seat reservations are not mandatory on the EC until Domodossola, the border point. I believe you cannot make reservations for the "domestic" leg to Domodossola, at least that's what the SBB app tells me.
There is a generally a second unit coupled until Domodossola so board that one to find more seats. Check the departure boards or SBB app.
It's true that these trains are quite crowded but I've always found a seat (and never made a reservation).
Seat reservations are not really a thing within Switzerland. I've lived here all my life and never made one ! It works in a first come first served basis but there are always seats if you board at the front/rear ends, not in the middle.
If you want more advice please let us know travel date and time. These trains are busier at weekends but there is an extra train on Saturday morning.
And if you can afford the extra hour definitely take the scenic Old Lötschberg Railway (hourly RE via Kandersteg, every other hour with a guaranteed connection to Domo in Brig).