One relatively simple and cheap-ish, potential fix is to use a much larger battery then what the Ruizu is supplied with. In other words, you can replace the battery it comes with one with far more capacity, or more easily, use a battery bank designed for phones. That is, rather than plugging a solar panel in, just plug a battery bank in to the USB connector, and off you go. No solar required, which as Harold has identified, could be where the problem is.
In order to get about a month it looks like you'll need about 8,500-10,000mAh of capacity, or ten times what the supplied battery provides. Something like this should work ok: https://tinyurl.com/y872gl64
And you should be able to get these in lots of different places, like office supply or phone stores. You can also get larger ones (e.g. https://www.officeworks.com.au/shop/officeworks/p/comsol-20000mah-dual-p...) if you want it to run for longer.
Could be worth a try anyway.