If you can’t find a partner to help test with, acquire or borrow second phone or SMS-capable cell device, i.e. a phone that has Bluetooth Extended (BTE) and an OS up to date enough to satisfy the goTenna system requirements but no longer has cell service. I do a LOT of my testing this way, leaving the extra paired device set at home and using the one in the vehicle to send test messages back to the one at the home location.
BTW, you doing exactly the right thing to maximize the reach and capability of your personal system setting up one at home as a relay that, in effect, serves as a powered antenna, and one on the vehicle that acts as a mobile repeater.
The only iffy point is that this won’t help for Shouts, which do not relay. Here’s where your post here can help if you add an appeal for folks to contact you with their GUIDs and/or publish your own GUID here, which will allow you to add them to your Contacts and enable you to take advantage of hopping through various nodes to extend the maximum reach of your efforts. Like with email, it helps a lot to have the address for someone else in order to facilitate the contact.
If you need to extend the range of your Shouts, the main practical option is to increase the height of your paired device, while staying within Bluetooth range. One option, although I didn’t originally design it for mobile use, is my FireFly BigStick design. I need to caution you need to do your own testing with the BigStick to determine if it’s suitable for mobile use as you will use it. I suspect mine is fine around town, but not sure about highway speeds. You also need to adjust the BigStick for your anticipated overhead clearance and judge things carefully as you encounter them or something could go bang…