For this having at least level 4 in ice fishing gives you the best chance at getting this achievement.
If you're level 4 and above you can get a smallmouth bass over 5kgs from fishing huts on lakes (Mystery Lake for example)
If you don't have level 4 or 5 in fishing you can try your luck with Coho Salmon in Coastal Highway as it is the only fish that is naturally more than 5kg without a high fishing level.