Problems with mobile provider

So basically I have made a huge mistake and I know its my fault for being dumb, but I wanna know if there is a way to somehow alleviate things. I signed a contract with SoftBank, I decided to contract Softbank 光 due to the fact that they offered a free router rental. As such, I decided to also change my phone provider so that it would be a little bit cheaper. The service of the internet and the data itself is fine. My main problem is with the customer service, they did not explain to me the 手数料 or 工事費 to me at all among other fees, I think the installation should be minimal since the apartment uses NTT.

There is cashback however I doubt I'll even get it since they got my name wrong even though I asked them to change it, and apparently I got very little cashback only 1万円.

When I went to the branch I made the contract at, they refused to attend me because the representative that attended me no longer was there so I basically have no on-site support. I've also gone to other branches and they refuse me support. I have tried getting in contact with support online to no avail and also on the phone. They basically are like we will explain it to you. But the problem is they are charging me way more than they said they would at the start and so I wanted to change provider.

The main question is, does someone with more experience in this matter know, if there is a way to change providers in such a way that I lose the least amount of money with all the charges like the 解約 fee and such and ideally not be tied to a contract? The problem with 格安 SIM is that I'd basically have to pay Softbank all the fees outright,

by Sharp_Ad6204