then how can the not have internet in that era? and still use pager? where's their Motorola tac cellular phone
Wut? There was no internet in 1991, at least not like there is today, that didn't really start until around 1994. Indeed, fun fact, Star Trek Generations, released in 1994 was the first movie ever to have its own website.
Why shouldn't they be using pagers? Pagers continued to be widely used even as cell phones began becoming commonplace in the mid and late 90s. The movie Volcano released in 1997 had a product placement deal to showcase a new model of pager released that year.
Motorola tac cellphones were still relatively new in 1991, having only been just released in 1989. But this show does make it clear NIS isn't well funded, so it stands to reason they're not getting the newest or best toys at this point.
Regardless, the date 1991 was mentioned in all this show's promotional material and was mentioned in onscreen dialogue and seen on surveillance tape footage until the more recent episodes began using 1992. Plus, if one follows NCIS lore, than Gibbs and his first wife get married in the 1980s and his daughter was born in 1984. Plus the Origins season finale features Lara Macy who was featured in the OG NCIS episode Legend (which was also the backdoor pilot for NCIS Los Angeles) where it was stated that Gibbs and Macy last met seventeen years earlier. Given that episode aired in 2009, that would mean they last met in 1992.
So, I really don't see where you got this idea the show is supposed to be set in the 1970s.