You will be able to get offline maps and associated functionality by having Pure Maps and OSM Scout Server installed on the same device. In this case, the server handles map storage, for example. Planet is not split into 20x20km sections, but larger regions, with the list of regions at https://github.com/rinigus/osmscout-server/tree/master/scripts/import/hierarchy .
You can test all already on desktop Linux (assuming that you don’t own Sailfish OS device) by installing OSM Scout Server and Pure Maps from Flathub. On desktop, start the server first, configure it (mainly state where you want to keep maps and few other options in the wizard), get maps and then start Pure Maps. To access all in offline mode, select offline mode in Pure Maps and you will be accessing maps from the server. Testing is limited if you don’t have it on mobile (with GPS), unfortunately. We use it on Sailfish in practice.
Server users’ guide is at https://rinigus.github.io/osmscout-server/en/
If you find an issue which is not filed, do so at Github. You are welcome to join and participate in development as well.