Ole Olsen Underbakke & Thorbjør Olsdatter Underbak

Hi, I am looking into what became of my 3rd great grandfather Ole Olsen Underbakke, who was born in 1809 in Bø, Telemark, Norway, and his daughter Thorbjør Olsdatter Underbakke, who was born in 1866 in Suldal, Rogaland. Ole and his family left Suldal, Rogaland around 11 April 1872 for America. The last record I found of him and Thorbjør was when the family arrived in Quebec on 1 June 1872. Does anyone know how they were likely to have traveled from Quebec to Iowa? In the church records for the Stavanger Lutheran Church in Ossian, Iowa, the name Ole Olsen Underbakke appears probably as new members of the church in the year of 1874, if I remember right. It is not clear if Ole Olsen Underbakke was still alive. FYI, he had a son by the same name who was born in 1842 and died in 1885 and is buried at the Stavanger Lutheran Church cemetery. According to the 1880 U.S. Census, Ole's wife Thorbjør Knutsdatter is listed as a widow living with a son Knute in Dover, Fayette, Iowa. So, Ole and daughter Thorbjør had to have passed away sometime between 1872 and 1880. Neither of them are not listed as being buried at the Stavanger Lutheran Church cemetery. They likely died somewhere between Quebec and Ossian, Iowa. Thorbjør was only 6 when she got to Quebec. Her parents were much older, 48 and 56, when she was born. I don't know if she was healthy or not. Does anyone have any suggestion how I can find them - when they died and where they are buried? Thanks.