True, the english site is too limited, and for the other sites I am at a loss for Hebrew, sorry. But that never stopped me before, just because I can’t read something does not mean that google translate and wikipedia will not make me dangerous
If I understand correctly Bezeq is the old monopoly teleco of isreal, and they use VDSL2@FTTC for their 100Mbps plans, so in all likelyhood you need a vdsl2 modem that allows bridge mode and will work with your ISP (unless your ISP router can be configured into bridged-mode).
Not really, for lantiq/intel/maxlinear modems there exists an open sourced driver, but you still need to load a proprietary frmware blob. That said a few of these devices are supported by OpenWrt, mosltly based on the lantiq xrx200 architecture, some of the them, like the BT HomeHub5A can be gotten relatively cheap on ebay, already flashed to run OpenWrt.
Could be; but I bet they already have customers that use other routers so they will probably be able to help you in either setting their modem to bridge-mode or (potentially unofficially) tell you which bridgable-modem is compatible with their service.