Backup the modems config to a place where you can find it back
In the config file, look for the section:
(protect
(udpflood
(enabled(1))
(rate_limit(30))
)
(icmpflood
(enabled(1))
(rate_limit(30))
)
(synflood
(enabled(0))
(rate_limit(30))
)
(winnuke(0))
)
set the udpflood "enabled(1)" to "enabled(0)", and reload the configuration into the router. It appears to remove the Steam issue (although the flood of packets still results in some NAT failures, probably due to the large amount of traffic). Try it out, and see if it works for you.
if it works you can try to enable and try a higher value in rate_limit(30))