Two OpenWrt serial connexion,ipv6 pd of ula and common ipv6

I get first openwrt working correctly with ipv6-pd from pppoe by isp, but second router with dhcpv6 client only pd ula from the first openwrt, how to do next to get public ipv6 behind second router? thanks

Do you really have PD working between you and your provider?
You masked too much, so it is not clear whether IPv6 and IPv6-PD on PPPOE_6 are from the same subnet or not.
Anyway, you will need a larger prefix delegated by the ISP, like /56.

no not the same /64
upper router

**Protocol:** Static address
**Uptime:** 2d 2h 21m 42s
**MAC:** C0:61:18:FC:B8:9C
**RX:** 738.98 MB (3446624 Pkts.)
**TX:** 7.17 GB (5731183 Pkts.)
**IPv6:** 240e:306:2783:61c1::1/64
**IPv6:** fd6a:b64f:a8e4::1/60
**Protocol:** Virtual dynamic interface (DHCPv6 client)
**Uptime:** 2d 2h 19m 57s
**IPv6:** 240e:306:2701:a634:xxxx:xxxx:xxxx:xxxx/64
**IPv6-PD:** 240e:306:2783:61c1::/64

downer router

**Protocol:** Static address
**Uptime:** 1h 34m 33s
**MAC:** 04:A1:51:A1:95:E3
**RX:** 22.33 MB (82692 Pkts.)
**TX:** 117.14 MB (111009 Pkts.)
**IPv6:** fd6a:b64f:a8e4:4::1/62
**IPv6:** fd92:30e9:6d9e::1/60

**Protocol:** DHCPv6 client
**Uptime:** 1h 34m 53s
**MAC:** 04:A1:51:A1:95:E4
**RX:** 113.32 MB (104975 Pkts.)
**TX:** 23.51 MB (76896 Pkts.)
**IPv6:** 240e:306:2783:61c1::xxx/128
**IPv6:** fd6a:b64f:a8e4::xxx/128
**IPv6:** 240e:306:2783:61c1:xxx:xxx:xxx:xxx/64
**IPv6:** fd6a:b64f:a8e4:0:xxxx:xxxx:xxxx:xxxx/64
**IPv6-PD:** fd6a:b64f:a8e4:4::/62

I would try asking the ISP for a larger prefix, but another option is switching the 2nd device to a dumb AP mode.

how to change openwrt to ap mode? and how about wire ethernet connextion?

does ndp proxy works ok?

The prefix from the ISP is itself a ULA. That's not useful. Also if it were a public PD (which it is not) it is only /62, which doesn't allow further delegation.

Unless you can change things at the ISP, all you can do is use it as the single GUA /64 from the ISP and using relay assignments with NDP relay/proxy instead of delegation and real routing.

No the upper router get /64 public v6 but the downer just get /48 ula which is set by first router. how to set relay mode?