Hi, new here and just sharing my SIM card lesson here in the States. Its a long post, but hope you take 5 minutes to read it, so you can save yourself 5 days of effort later. I have to say thanks to other members for the useful information they have posted about SIM cards and data plans. My first attempt at getting a sim running in 1 of my 2 RS3 receivers was using a $25/mo ATT tablet data plan sim from one of my Samsung tablets which struck out and never worked. The second ATT SIM I used was pulled from my Phone and is the lowest normal data+voice plan at $65/mo which worked instantly. The third SIM I got working was an ATT prepaid card I picked up from Walgreens. I initially got it running by registering it with the IMEI from an old phone, but it did not work in the RS3. Per recommendation from another post here I called them (in this case prepaid support) and over the phone we changed the IMEI# to the RS3 modem’s IMEI and also turned it into a data only prepaid plan and to my surprise that did work, and the experience was not terrible, good even. I used that card for a couple weeks with great success, but then I did more reading DAC, and this is what I have to say about that experience…
https://www.dataactivationcenter.com/
Hands down the best SIM card and data plan setup experience of my entire life. So, the story is my experience has been so good with the RS3’s that I leant them to good friend who ordered two of his own before he left the first project site that he used them on. Trying to get him setup with a SIM I recommend we try DAC for both the base and rover as he only had Verizon on his iphone and had spotty coverage for the rovers ntrip corrections over bluetooth. We went to the DAC site and looked at all the plans and then hit the get started button, but realized we didn’t have any SIMs to activate yet. Simply use the live chat and ask for them to ship you the number of SIMs you need. A few days later with SIMs in hand we filled out the activation form for two lines using their DAC3 Pro multicarrier 1GB/mo plans for $25. Popped in the cards and started trying to setup the mobile data usage from Emlid Flow, got a call from DAC with Amanda who guided us through the rest of the process with ease. I will skip over the some of the finer details but needless to say my buddies RS3s were working fine and the day was done. The next day Amanda called me back already knowing I was going to setup my cards next and instead of filling out the form she just had me read out the IMEI and serials from the RS3 which you can find in Emlid flow app when connected to each device. She filled out the rest of the information and setup my plan just like my friends per my request in just a few minutes. After a brief 10-15min for those SIMs to register on the network I was in business too! Quick note, turn on Roaming, and their DAC3 Pro APN is simply DAC.
Aside how easy this was as compared to the previous trial and error you get 3 carriers at once (ATT, Verizon, & T-Mobile) while having just about the best experience you could hope for doing it. The online form is intuitive and simple, but the people are proactive and will call you up and help you out quickly and cleanly while being just about the most professional and well-spoken I have ever encountered.
Lastly, I got a text and an attempted call from the owner of DAC today (Kelly) who just wanted to touch base and make an additional connection with me if I needed it. At this point I am simply jaw dropped and impressed. With all the various robots, computers, drones, and now GNSS receivers I have needed mobile data I find myself asking, “where have you (DAC) been all my life” 
Last tip: https://map.coveragemap.com/ change the map to signal strength and select your carrier).
Hope this helps