Currently the Banana Pi R3 seems to be the best option for a pplznet device. It is avaivable between 100 and 200$ and offers two fiber ports. Thus it has the minimum amounts of fiber ports to connect to the neighbors.
There is already an open pull request so the next OpenWrt version probably will be supported.
https://banana-pi.org/en/bananapi-router/99.html
https://wiki.banana-pi.org/Getting_Started_with_BPI-R3
https://forum.openwrt.org/t/how-to-flash-emmc-on-banana-pi-r3/137418/2
https://github.com/openwrt/openwrt/commit/a96382c1bb204698cd43e82193877c10e4b63027