My primary area of study is Mennonites and Mennonite Cultural History.
It is important to note at the onset that the Amish are not a monolithic group. There are extreme variables between different settlements (Lancaster compared to New York compared to Oklahoma, for example) and between districts within a settlement. A significant part of this is that Ordnung, the rule that governs Amish life, is purposefully kept as oral tradition and not written down and codified (that would be, in their view, too legalistic). Each bishop in charge of a district is left to interpret the oral Ordnung himself, though with a consultation with nearby bishops if so desired.
It is not correct to say that the Amish stopped adopting new technology. Instead, they choose to interrogate how any new development will impact their community. Gerald Mast noted in a presentation at the Amish COnference hosting at the Young Center for Anabaptist and Pietist Studies at Elizabethtown College in 2016 that the Amish work with two axioms: change is bad AND what we already do is good. This is in specific opposition to the modern assumption that changes--especially in communication--are at worst neutral. On that same panel, Lindsay EMs noted that the current trend in Amish decision was changing from a "fence" model expounded by Donald Kraybill in The Amish (2001) where rules are made to separate a technology from individuals to a "switch" model where the decision making is instead about controlling information.
The second part of the question is more complicated. When they split from the Swiss Brethren in 1693, that was an issue of enforcement of church discipline not one of use of technology, so they would have been "modern." One point of time to highlight would be the emergence of the Old Order movement, common across Mennonites and Amish, ranging roughly from 1845-1900. The main points of disagreements here would have been Sunday Schools and Meetinghouses, with cars entering the discussion later. This would have seen the split between the Old Order Amish (such as the Amish in Lancaster County) and the Beachy Amish/Amish Mennonites, who for the most part gradually merged with the (Old) Mennonite Church
As a way to tie this all together, I will turn to the Lancaster Settlement, which is the one I am most familiar with, and give a brief description of how they have negotiated with technology. First, Lancaster is one of the most conservative settlements when it comes to theology, but one of the most liberal when it comes to using of technology. In some sense, this was forced upon them by the Amish farmland crisis in the 1960s and 70s, where farmland became too expensive in the county for parents to buy each child a farm when they were married as had been the custom previously. The choice they faced was emmigration and adaption. TO be sure, some choose to resettle elsewhere, but others choose to adapt to light manufacturing and other nonfarming employment. I say nonfarming lightly, because most still live on a farm, but farming does not provide their primary income--growing flowers in greenhouses is a major economic opportunity that is controlled by Amish women, and interesting studies have been done on that (Beth Graybill and Diane Zimmerman Umble). To accomodate the needs of the business, the lines of technology blurred--first telephone, then computers and the internet became accepted in places of business because it was neccessary for economic survival. However, these are still forbidden in the home because it is felt that would be detrimental to the community. Similarly, it is forbidden to connect to the electric grid, because that means they would no longer be self-reliant. But that same reasoning allows for diesel generators and solar panels.
Sources and more detail can be provided upon request. Good starting points are
Kraybill, Donald B., Karen M. Johnson-Weiner, and Steven M. Nolt. The Amish. Johns Hopkins University Press. 2013 (Paperback)
Nolt, Steven M., The Amish: A Concise Introduction. Johns Hopkins University Press. 2016
Ruth, John L. The Earth Is the Lord's: A Narrative History of Lancaster Mennonite Conference. Herald Press. 2001 [Out of Print]
It would be helpful if as part of an answer someone could get into how the Amish (as in, Pennsylvania) decide where the line is on which technology they've decided is "kosher" since they first settled in the US.