if these are tunneling interfaces, then simply use interface name instead of GW IP:
/ip route add routing-mark=channel1 gateway=ModemA
/ip route add routing-mark=channel2 gateway=ModemB
if it's broadcast interfaces, then use the following syntax:
/ip route add routing-mark=channel1 gateway=188.8.131.52%ModemA
/ip route add routing-mark=channel2 gateway=184.108.40.206%ModemB
for second case in v3 you will need to have routing-test package installed
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