My PPPoE customers can't reach between them

Hello. I’m going crazy, and maybe it’s a little thing that I can’t see.

I have 4 PPPoE customers. Each one, have a static CG-NAT IP in their profiles, for example:

C1: 100.64.0.10
C2: 100.64.0.11
C3: 100.64.0.12
C4: 100.64.0.13

Customers can navigate to internet and perfect. The problem comes when C1 want to connect to C4. All customers can ping gateway, but not between them.
Each PPPoE customers have a independent route rule. I have tested NAT, Mangle rules, Firewall rules, nothing worked.

Anyone can give me some light for make my PPPoE customers be reachable between them? Thanks!!!

A traceroute from a customer:

traceroute to 100.64.0.31 (100.64.0.31), 30 hops max, 38 byte packets
1 10.15.0.1 (10.15.0.1) 37.661 ms 31.160 ms 38.645 ms
2 * * *

10.15.0.1 is the PPPoE gateway of all customers.

Thanks!!