Being familiar with the guy who is Kombi Life I know that he is not a USA citizen and lives in Europe. So knowing where his citizenship is meant that the Teltonika services and devises were unlikely to have much of a presence in North America. A few seconds of research proved that my suspicions about it not being USA based were accurate. That company is based in Lithuania.
My advice is for the best convenience for your use try to find something local where you can get any devices you might buy serviced in the USA. Shipping cost to and from Europe is definitely not affordable and your warranty on physical devices be questionable as to getting items replaced.
As to SIM cards they are very common in devices such as phones, tablets and other cellular connected devices including various models of laptops. Also some devices now use E-SIMS instead of a physical SIM chip you insert into the devise. If you do not know what an E-SIM is look it up, they have become fairly common now.
I can easily access the internet while traveling using my cell phone. My cell phone data plan lets me use my cell phone as a hotspot. I needed to do that today so I could connect my laptop to the internet to go into my email account and download some files from that account onto my laptop. Since I am self employed with an online business it a great convenience for me and a space saver just to have my phone be my hotspot for my other devices I might want to put online. That means I also do not need a Wi-Fi router in my travel trailer. I have a little USB powered Wi-Fi router but in 3 years of full time nomad life I have never needed it.