I used to have connectivity and data speed issues at music festivals and large conventions, but both permanent infrastructure at convention centres and temporary towers at parks used for festivals have mitigated this (in my experience).
In both cases the event is planned many months in advance, and there are incentives for both event organisers and telcos to ensure phone subscribers have a good experience (avoids “reception was terrible at Glastonbury” or “I couldn’t make any calls on Vodafone, I should switch network”).
But protests typically lack the timeframe, and (I expect) a sympathetic ear from the telcos. Even if the timeframe is know well in advance, gaining permits to stand up towers (and associated labour, equipment rental) costs money, and spending the money signals implicit support for the cause.
While load sharing among neighbouring towers could alleviate minor increases in traffic (maybe 2x) I can’t see the networks over provisioning the radio and back haul for permanent infrastructure to support a possible 10x surge and then let it sit 90% idle. Dedicated fiber leased lines are expensive.
Maybe protests should instead be organised near convention centres which already have capacity for surge traffic?