from my past experience,..
when after flashing the wrong firmware causing the purple led gone,..
i notice when after freshly flashing,... the first time u insert sim and plug into computer or router,.. the first connection is always meant to connected to 900MHz,..
since flashing wrong firmwareis no big deal when u could use older dashboard ( come with old DC-HSPA+ capable driver ) or router ( where u can manually input the modem bin file for DC-HSPA+ capable ),....or openwrt router ( which come together with DC-HSPA+ driver in the openwrt firmware )
so no big deal for the purple led as long as the modem itself can performs
currently ZTE MF820D is the most worth value LTE modem,..

using zte mf820D + openwrt router, u can directly use AT Command to force to LTE 1800 or 2600,..