The only change to DLM an engineer can make is a reset. That's what your engineer did. However, after a reset the DSLAM is back in control and will change DLM to suit your line. There's nothing they can do to change that.
Each ISP has a choice between two or three profiles controlled by DLM.
1. Speed. Delivers the highest speed possible. The thing with this is if there's errors the speed won't be reduced so much but the connection will suffer from mega retransmits to mitigate the errors.
2. Reliability. Speed gets reduced and greater use of interleaving to mitigate errors.
3. Blend. Mixed use of both of the above to balance performance and reliability.
I'm a mile from the CABINET that served me. When I was on TalkTalk I had the speed profile. 75/20Mbits but seemed really slow - because of all the errors - retransmit taking a lot of that bandwidth. Then I moved to Vodafone with their reliability policy. 65/18 Mbits but it felt a whole lot nippier.
The bit rate does not dictate the actual experience of the end user. Vodafone won't change their policy for just one or one thousand customers because it's contractual with openreach - one or the other, no changes.
To be honest the overall performance of the 3 policies are very similar but is gained by different methods. However, with it's mega retransmit the Speed profile can actually feel laggy - which it is. Most end users are impressed by headline speeds without knowing what is involved in delivering that speed number. It's just a number that has nothing to do with the actual experience.
The laggy feel of your connection is not down to DLM. Look elsewhere.
