Would Openwrt let me use a /31 subnet mask if stock doesn't?

The reason is my ISP uses a /31 subnet mask for static ip and if I want a /30 it jumps from $15 to $25 a month. I want to setup up two networks on one internet connection and found a deal on a Linksys EA7300. I was going to use it as the first router but it refuses the /31 subnet mask. Would installing OpenWrt let me use the /31 subnet or should I return it and look for another router that can. Also do the routers behind the first also need the /31 subnet or can I use any subnet mask on them?

Do we talk about IPv6 or IPv4 here?

If we talk about IPv4 then the subnet is not a hardware limit but a software limit and I haven’t seen any subnet limits upstream or downstream to OpenWrt to this day.
Every single internal network you setup can also have it’s own subnet definition.

But do we talk about IPv6, then the concept of subnet are more complicated!

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This is what I was hoping to hear. I didn't want to use the old ISP Actiontec F2250 as the first router since the EA7300 has twice the ram/CPU and I assume it will be doing more work than the secondary ones. Glad I chose to look for a router with OpenWrt since you never know what you will run into. And yes its IPv4.

filogic are dropping in price, might be longer term deal
https://openwrt.org/toh/views/toh_available_16128_ax-wifi