>>106035950
router1 does not have a routing table entry to forward packets destined to PC2's IP to router2.

Router1's default gateway is set to forward everything to internet.
Router2's default gateway is set to forward everything to Router1.
PC2 sends ping to Router2. Router2 forwards everyhing to Router1. Router1 knows PC1, forwards the ping to PC1.
PC1 sends ping to Router1. Router1 doesnt know about PC2. Router1 sends to internet.

To fix it, set a static routing table entry for 10.1.2.0/24 in Router1 to forward to Router2's IP, assuming PC2's IP is something like 10.1.2.3.