Does the Samsung Galaxy 5 support Band 12 with AOSP/CM13?
No, it doesn't have the hardware
Here is the T-Mobile spec sheet for the Galaxy S5 that seems to indicate it doesn't support Band 12.
Network 2G/3G/4G/LTE Capable UMTS: Band V (850); UMTS/HSPA+: AWS Band IV / 2100 / 1900 / 850;Band II (1900);Quad Band GSM;LTE GSM/GPRS/EDGE: 850/900/1800/1900MHz
And, here is the Spec Sheet from Samsung that seems to indicate it doesn't support Band 12,
Network FREQUENCIES AND DATA TYPE LTE: Bands 1/2/3/5/7/8/4/17 HSPA+/UMTS: 850/AWS/1900/2100MHz GSM: 850/900/1800/1900MHz