Since Apple used propriety technology for extending their AirPort base stations, only other Apple base stations work best for extending the wireless range of a primary base station ... especially if you want that extender to use a wireless connection to the primary station.
FYI, Apple stopped selling AirPorts back in early 2018, so it has been awhile. Regardless, they still support them ... just not sell them. You should still find them available at places like eBay, but doing so comes at some risk as they are fairly outdated devices.
Does your Solar Cell unit have Ethernet connectivity? If so, you may want to consider getting a wireless bridge. These connect to an existing Wi-Fi network and provide a connection to an Ethernet device. One example of these is the TP-Link AC750. This device is actually multi-purpose as it can perform as an Wi-Fi extender (for non-Apple routers), a wireless Ethernet bridge, and as a "travel" router.