DNA can help, but it is not guaranteed that the unknown gggrandfather’s descendants have taken DNA tests. If they are from the UK and their descendants stayed in the UK you are not likely to have as many matches as those who emigrated to the USA for example. It might just be a waiting game for you.
Hints and tips and success stories are good to hear and may help, but what works for one might not work for others. It can sound so simple in theory, but the reality can be different and quite disheartening.
If you took a test with Ancestry, as most people seem to have done, upload your raw data to other sites such as My Heritage, and pay to unlock their useful features. You might find you have different matches on different sites. As you probably know, Ancestry do not return the favour and allow people who have tested elsewhere to upload their raw data to Ancestry, so you might miss out on some matches. Ancestry also do not have a chromosome browser, which others do, and which you might find is a useful tool.
Good luck.