Intern becoming QA Engineer who feels cheated on new contract

Hi everyone, This is my first post and it's an import one to me. I was originally hired as an intern for a tech company to walk around my office space w/ a mobile device collecting data packets every meter. I did this repeatedly several times a day for several weeks. I was getting paid $15 / hr. At the the time I thought it was fair seeing as what I was doing didn't involve much work and was a small task. I also only have a certificate in C++, no college degree and took 2-3 years or Java, Python, HTML and C++ in high school and college before dropping out to go into tech. Fast forward to today. My internship ended and they rehired me as a full time employee. However, it took them 42 days to issue me a new contract (42 days I worked. I will get paid at my new rate for only 12 of those days). Now that I'm a full time employee they have me flying to Nevada, North Carolina (7 times), Texas, and Arizona collecting hundreds of data points that take several days and dozens of hours to collect. I walk up to 9 miles a day and 30 flights of stairs collecting these data packets from AP units sending wifi and BLE. Then I also manage these accounts with the clients to make sure they are happy. The accounts I'm working with and trying to sell are multimillion dollar contracts (usually $2-8 million). After I collect hundreds of hours of data I then run all of it through Matlab and generate dozens of readable measurements that I then publish into neatly designed spreadsheets. Then the team makes the changes they need in the code and I'm on the next flight back to the client to collect more data. In addition to all that I also make sure these multimillion dollar deployments were done perfectly and if any hardware was installed incorrectly I fix the problem. If our apps are having any issues I'm the first to report them and have them fixed. I guess you could call me quality assurance but I have an engineering position in the company. TL;DR: Here's the issue, they only want to pay me $18 / hr to be a Test Engineer for them. I know money is not an issue for this start up and I think I should be compensated between $25-45 for what they make me do. What are your opinions on the matter? Should I not sign the new contract and ask for better compensation and stock? What do other people in QA And test engineering get compensated roughly at other company's?