tile cache

I am quite new in mapping, try to work with JOSM under Ubuntu 18.04. I have installed JOSM succesfully, it works so far. My problem is, that the bing tiles (at present I use nothing else), once downloaded, dissapeare after the file has been saved, i.e. the next day I have to download all the tiles again. I have to work independent from the internet, so a lasting cache is essential for me.

I have studied the JOSM / OSM documentation, the forum, but there is nothing what really helps me. No where it is clearly stated how the cache is managed. I have also increased the cache size, that seems not to be the problem. I have tried to set up mapproxy, but the configuration is too complicated for me.

Are there simple solutions?