This is a constant issue for many travellers and there are a number of ways to keep your electronics charged.
What I need
I keep my phone, iPad, GPS, Kindle, bike lights, head torch, and shaver charged. (yes, a shaver. I look even more homeless with a grey beard) Most of these don’t need charging every day.
My solution
I have two 10k mAh batteries. One of them is down to half capacity but the good one will charge my phone at least twice. This means I can go for a few days without power. If I don’t need connectivity, I’ll use aeroplane mode and / or extreme battery saver on my Pixel Phone.
I’ll charge up when I’m at a BnB or hostel. Even campgrounds let you charge devices but the security of your device can be an issue while it’s charging in a toilet block or kitchen area.
When I charge, I have an travel adaptor with multiple USB-A and USB-C ports. It can fast charge one device at a time. I can also plug my Australian charger into the back of it and use its two additional ports. The beauty of this, is that I can fast charge an additional device at the same time, and this is perfect for camping grounds and hostels when power points are in short supply.
Alternatives
I tried a solar panel a few times but it struggled to charge the old battery. I now realise the issue was probably the battery, so I may look at them again and then update this post.
Dynamos are popular, with hub based dynamos used by lots of bikepackers that I meet. There are other bits needed in addition to the hub dynamo, and many are connected to permanently attached lights. I opted not to do this when I got my bike. I felt the whole setup was too complicated and therefore likely to fail during my multi year trip. I also didn’t want permanent lights attached.
I rarely need lights while riding except for the rear when I’m on a busy road, and good rechargeable lights have been sufficient over the last couple of years. I’ve always got my head torch as backup.