Judging by his first message https://groups.google.com/forum/#!original/alt.fan.pratchett... , connectivity was via demon so I'm guessing from the date (Demon launched on the 1st July 92) he was one the first tenner a month subscribers. I seem to remember that his cix account had something along the lines of "Yes I am that Terry Pratchett" in the equivalent to the finger command.

Sadly demon as a brand has been killed by it's current owners https://www.theregister.co.uk/2019/01/11/vodafone_to_shutter... .

I think I got on the internet properly a year or so later, although I had access via various means early than that.