It's my understanding that R530C is almost same hardware as R530U (US Cellular) and i535 (Verizon). It's a known thing that both R530U and i535 are capable of connecting to GSM networks. See threads here:
SCH-530U US Cellular on Tmobile 4G LTE (HELP) - xda-developers

and here:
Unlock Verizon Galaxy S3 i535 as World GSM Phone - simplified method - GSM-Forum

Has there been any success with R530C? It seems we have 2 problems here:

1. Phone does not recognize foreign SIM cards. This is not a hardware limitation. I have seen it recognizing T-Mobile SIM on some firmware, which I cannot find anymore. If anyone can see "T-Mobile" when t-mo SIM is plugged in, can you please post yourt firmware version?

2. The "GSM" option can not be activated using phone settings. I think it's possible to change by turning the phone into service programming mode and using QPST or DFS on it.

Can anyone comment?..