the overview of my situation can be seen here:
I have multiple overlapping networks here, namely 10.0.0.0/8 and 10.64.5.0/24 and 10.64.95.0/24. I've decided to separate them via NAT - using 192.168.88.0 network. The other NAT ("Red") works fine and separates another subnet. R2 and R3 are Mikrotik routers, their respective configs:
R1 is a SOHO TP-LINK MR3220, which is default GW for the machines in pink (10.0.0.0/8 subnet). It serves as an Internet gateway for them. The problem is, I can ping 192.168.88.1 / 2 / 3 (in essence, R2 interface) from "blue" IPs (192.168.40.71), but cannot ping 192.168.40.71 from "pink" machines - the ping is sent to their default GW (10.0.0.1 = TPLink), and the router in turn has static route set - 192.168.40.0 via 10.64.203.88. However, the traceroute to 192.168.88.4 (R3 interface) stops at R2 (10.64.203.88).
I can ping 192.168.88.1 from 192.168.40.71 (blue), but not the other way around. Mikrotik labeled R2 can ping 40.71 however.
All firewalls are disabled.
I suspect I messed something up in the default GW/static routes, but do not know where.