Let me tell you a story about the FRER-- when I was pregnant with my daughter, I took 3 FRER tests 12-14 days after my missed period-- ALL 3 NEGATIVE. Even the dollar store one was negative. 2 weeks later I took a blood test at the doctor's just in case something else was causing my missed period (I had no implantation bleeding and my symptoms were on and off). Got the positive via blood test and took a Clear Blue digi 2 days later that showed "Pregnant 3+ weeks". I had conceived around April 1 2016, and I took the tests April 24-26. My doctor chucked it up to my body not producing enough hCG for it to be caught on home test.
When I was pregnant with my son though in Feb 2018, I got a positive a few days before I even missed my period.
Keep testing or even go to the doctor's to get a blood test if you still haven't missed a period.