I tried enabling IGMP snooping on 18.06.4 and this didn't seem to work, so I decided to try the 19.07 SNAPSHOT that has Luci included instead:
https://downloads.openwrt.org/releases/19.07-SNAPSHOT/targets/
Glad to say that adding option igmp_snooping '1' to /etc/config/network under the LAN interface fixed this for me:
config interface 'lan'
option type 'bridge'
option igmp_snooping '1'
I also disabled legacy 802.11b rates (which can be done in the Luci GUI under "Network>Wireless>Edit>Advanced Settings" or by adding option legacy_rates '0' to /etc/config/wireless for the wifi-device):
config wifi-device 'radio0'
option type 'mac80211'
option hwmode '11g'
option path 'platform/10180000.wmac'
option country '00'
option legacy_rates '0'
option htmode 'HT20'
option channel '11'
Relevant Wiki links:
https://openwrt.org/docs/guide-user/network/wan/udp_multicast
https://openwrt.org/docs/guide-user/network/wifi/dumbap
Thanks @martin8801!