Travel / Mobile internet for tourists in Poland? [35]

Merged: Need prepaid mobile internet for one month in Poland. I will cycle the coast Świnoujście to Gdańsk

Good morning.
In june I will go by bicycle along the coast from Świnoujście (old name: Swinemünde) to Gdańsk (old name: Danzig).
These are my first vacancies in poland - so I am completely unexperienced ;-(

So I need polish mobile internet for four weeks only.
It should be reliable in rural/ coastal areas as well as in the cities.
And, if possible, low cost, since I don`t have much money.

Are there prepaid SIM cards (four weeks only) with
- most use internet, little use for phonecalls inside poland
- if cheaper, internet only?

If yes, where to buy?

Thank you in advance.
Karl