I had good luck with free wifi at coffee shops. The fastest wifi I had in Japan was at a Starbucks. I haven’t had great luck with the free wifi on trains at all.

I have heard Ubigo is a good option for esim cards. Make sure to enroll when you have reliable internet access already.

If you’re looking for a vendor for your portable wifi, IVideo gave us a router with excellent speeds on my last visit, and the return process was a simple prepaid envelope. Make sure to double-check the opening hours of the shop if you pick up your router locally.