I pay just over 1000 baht/month ($35) for True cable and high-speed internet. I live in Central Bangkok, reliability is good. True has cheaper packages. In Chiang Mai I used SiNet (fiber), I think it's 600 just for internet. Previously I had 3BB in Bangkok for I think 750/month. Some buildings will have that available already, or you can have it installed. Only SiNet will do month-to-month, True and 3BB require six month contracts. In a serviced apt or hotel or something you can rent for 2 months it's unlikely the owner/manager will let you install anything, but some places are wired already. You have to ask a lot of questions sometimes about the internet setup.

AirBnB places usually have dedicated internet in the condo or apt already. That would be my choice for a 2-month stay. Or a serviced apt that has dedicated internet. Lots of buildings have shared Wi-Fi either free or cheap, like what you get in a hotel. I've never been able to use those, they are usually slow and unreliable and make you log in all the time.