Zerotier "nested" problem

I have two openwrt routers in different places called ‘A’ and ‘B’.
They all have zerotier installed and can access each other.

There are two computers ‘a’ and ‘b’ under router ‘A’ and ‘B’ respectively.

Since the performance of routers ‘A’ and ‘B’ is not very good (MT7621), the connection speed can only reach 8mbps. So I want to install zerotier on computers ‘a’ and ‘b’ at the same time to speed up the connection.

But the zerotier on the computer cannot connect to each other by p2p. The zerotier on the computer seem to think that they are in the same LAN and are connected through the zerotier of the router.

I read this thread and add a firewall rule to drop packet but has no effect.

Is there any way to force zerotier on the computer to make p2p connection?
Or is there any other way to speed up the connection?