Definitely portal fantasy. So is Narnia.
Isekai and portal fantasy are almost synonymous I think. Isekai would probably be the umbrella term and portal fantasy is just a type of way (by some portal) to get to the isekai (other world).
You can also get to an isekai by reincarnation which is extremely popular 
Isekai is just about going to another world.
The “i” kanji is for “different” or “other”. “sekai” means “world”.
