Accidentally discovered this, with the old SIM card removed, restart phone without the new SIM installed. Then go through website prompts.
The explanation/process on the site to register the new 5G SIM card as replacement for existing 5G compatible Ting phone didn't work. Instructions on SIM card envelope didn't work. I got stuck in a circular loop that kept going back to: let's see if your phone is compatible, (my currently active phone was never a choice on the list, only older phones) "add new phone" entered imei number, "that phone is already in our system, let's go to device settings." Useless.
Can I also say, it's ridiculous that new Sim cards weren't just mailed out.
Be proactive. Why wait for me to contact tech support? Waste my time and there's after having connectivity wonkiness for weeks only to finally learn, "oh, well you have an old 3G SIM card." It's the same one Ting provided with the phone when I bought it. I'm an active subscriber, Ting has all the relevant information for the various devices and cards that I use. Just mail the new one before I need it.