Sony Ericsson W300i repair or not

I’m going to presume you had this SIM unlocked and moved it to a carrier that still supports 2G/2.5G networks since AT&T killed off theirs in 2017.

I’m going to say maybe - but look for a newer phone with 4G as the baseline if at all possible. In the US even 2G is on borrowed time with the only carrier left supporting it being T-Mobile, and even they’re depreciating it and then a full decommission in 2020. You can find 4G flip phones, but you need to be sure since a good chunk are still on 3G and that’s also about as dead as 2G is within a few more years. It may very well be a different story in other countries, but most of us are from the US and base our answers around what we readily know.