there are numerous posts like yours which you can find using search here is one which explains https://community.bt.com/t5/BT-Fibre-broadband/BT-speed-down-after-recontract/m-p/1870949#M273892%2F...
your attainable is still 54mb so maintain connection and monitor to see if speed moves nearer attainable if still problems or drops then post back
I have been patient and waited the 10 days but my speed has not increased to what it was before the contract renewal.
BT Hub 6A
10 Days, 5 Hours 32 Minutes 49 Seconds
8.95 Mbps / 44.62 Mbps
Maximum data rate:
9372 / 53286
6.5 dB / 5.8 dB
VPI / VCI: