its 2019 and I am facing exactly similar problem.
Been using this 1098/- plan for 4-5 months. It worked like a charm for all this time.
I used to get 12-14Mbps all the time. Makes 4K life easier.
But now.
I barely get 0.5 Mbps. Nothing changed as far as my location and device is concerned.
Yes, speeds improve marginally right after 12 am till 6-7 am but then again moves to that dreadful slow 0.5Mbps.
I tried tweeting about it. Got 3 calls from BSNL office but no resolution as such.
CC calls never connects. Always get the same error "sorry the network is congested".
Feeling slightly irritated.
What did you do during your time. Did it ever solved for you?