Can't connect to my one of the subnets

Hi everyone!
Can anyone help me with iptables commands?

I have troubles with connection within second subnet. Any addresses i tried to connect to first subnet gives me timeout. I think there is a trouble with the fact i cannot set up this command from the tailscale article in the both unraid servers:

iptables -t mangle -A FORWARD -i tailscale0 -o eth0 -p tcp -m tcp \
--tcp-flags SYN,RST SYN -j TCPMSS --clamp-mss-to-pmtu

Anyone can help me with this? I see what tailscale0 adapter is obviously not set up because the client is set up in docker.
How do i need to change the command to work this way? Maybe it is needed to setup in router somehow?

For info: static routes are set up in both routers (not servers), first subnet have OpenWRT router, second is Huawei (the model is highly modified for provider, but the firmware have lots of settings). If there is the issue - i can provide more info which settings i used.

Anyone? I still struggle with this problem. Maybe i did something wrong in unraid settings?
I understand, my problem is really not common, but i can give any info to you guys, just ask, please.