The one and only question that answers your question while answering me : do you have a working Internet connection ?
If yes, then nearly all is fine, and you can stop looking, as you've already mentioned what your current situation is : its doesn't break your internet access if your WAN IP is a RFC1918.
But you can probably forget about NATting so you can make internal (on the pfSense LANs) devices accessible from the Internet, as you have no access to the ISP equipment to do so.
If your "TP-Link Archer VR300" is truly working as a modem, its just converting POTS VDL signals to "Ethernet" signals and it doesn't do routing , firewalling etc. Its not the "TP-Link Archer VR300" that has a WAN, and a DHCP server that gives you the "10.101.37.22" pfSense WAN IP : this "10.101.37.22" comes from way up, somewhere from the ISP.
Why they do so ? There is the classic $$$ rule : they have no more free routable IPs left as IPv4 free available stock has been sold out meany year ago, and what's left has a huge price tag. Its seen before ; you want a real routable IPv4 ? You $$$ or €€€.