you can go and try different older versions of the app, they are all available online and you will find one that works hopefuly with the 7.
On the side note, if you can afford 2 hero 7 blacks, you can afford a phone that runs newer android. Even my 2 years old xiaomi 4a that i bought for 100dollars works ok with the newest Gopro app. It sucks for editing purposes tho but i have a pc for that anyways
Gopro Hero 7 Black
Gopro Hero 3 Black
Gopro Hero 5 Black / Gopro Hero 3+ Silver / Feiyu G5 gimbal - Sold