There are all sorts of reasons why actual transfer speeds vary from time to time. There's no reason to assume that the firmware change was responsible. What were the connection speeds before and after?
adslmax DOWNSince 04 Jun 2015 12:08 12.1 16.6 79999 20000 104779 36319 12.4 -0.4 0 0 11.5 0.0 0 0 0 0 0 0 16 8 46.00 47.00 0.00 0.00 UP: 0d 0h 36m DSW32 Plusnet Huawei
What is bridge mode? Does it mean it using VDSL modem only and connection to other router?