Free OpenSource VPN for Android, no bandwith limit, no ads, no logging. Basic but trustworth VPN , good for occasional use. Techradar review https://www.techradar.com/news/calyx-is-this-free-and-unlimited-vpn-worth-downloading ‘Calyx uses an open-source OpenVPN-based app from Bitmask, developed by the LEAP Encryption Access Project (another non-profit.)’
Alternative ProtonVPN Free, also OpenSource (freemium), no ads, no logging, no bandwith limit, but only permit the use in one dispositive (mobile or PC) in the free version.