You'll probably need IGMP proxying enabled if you use a multicast streaming television service with a YouView, BT or SkyQ box setup or something similar. If you don't have a viewing box like these, disable it and see if anything breaks. Services to devices like an Apple TV / Roku / Amazon Firestick don't need it enabled.
It's good that switching off SIP ALG helped. One site I manage had a Netgear D7000 and I disabled SIP ALG without any adverse effects. The four Snom VoIP phones there work fine.
I don't like the Netgear routers because they lack the ability to do basic firewalling such as blocking or allowing specific IP addresses and ranges. Drayteks can do this but I usually use a dedicated firewall between the router and the LAN.