You can use a Verizon SIM - iPad - View countries with supported LTE networks - Apple. Verizon is not a participant in the universal Apple SIM program, so you will need to get a Verizon SIM to use from Verizon and install it in the nano-SIM slot on the side (Apple SIM - Apple)
That is why even with the Apple embedded SIM, the iPad Pro includes an accessible SIM slot on the side. Also, if you use the embedded SIM with AT&T, the SIM will lock to AT&T so after that to switch carriers, you’d need to use the side SIM slot (AT&T locks the Apple SIM to their network, not the device itself).