Is the IP of the huawei modem 192.168.8.1 and your WAN on MikroTik is from the range 192.168.8.0/24? And what is your LAN address range?
1) your huawei modem is 192.168.8.1
2) your WAN has to be in range 192.168.8.0/24
3) your LAN is 10.1.1.0/24
then put this in your MikroTik's console:
/ip firewall filter add chain=forward src-address=10.1.1.0/24 dst-address=192.168.8.1 action=drop
However, if you do this, you won't be able to access modem's web interface as well! To avoid that, you have disable that rule everytime before you want to log in, or add accepting rule BEFORE you put the above. Otherwise, you have to place accepting rule as first:
/ip firewall filter add chain=forward src-address=ALLOWED_IP_ADDRESS dst-address=192.168.8.1 action=accept
After all it has to look like this:
[admin@MikroTik] > /ip firewall filter print
Flags: X - disabled, I - invalid, D - dynamic
1 chain=forward action=accept src-address=YOUR_ALLOWED_IP dst-address=192.168.8.1 log=no log-prefix=""
2 chain=forward action=drop src-address=10.1.1.0/24 dst-address=192.168.8.1 log=no log-prefix=""
YOUR_ALLOWED_IP is of course the IP of the computer that you want TO GIVE access to your modem's web interface.