Openwrt: access ftth modem after pppoe

is it possible to access the network equipment that is "in front" of a pppoe connection?
i have some network stuff "before" openwrt router , which is connecting via pppoe connection to the internet.

the diagram would be:
ONT -- SXT1 <--> SXT2 -- Openwrt router (pppoe).
ont and sxt1 are on a different location than sxt2 and openwrt router.
is such a thing even possible? or should i connect in some other way so that i'm able to access both sxt devices? i need access to them to periodically check if the wifi link is good with no interference.

That should be possible, in the GUI create a new interface with the correct ethernet interface and VLAN (if any) assign a static address that will allow you to reach the upstrem devices (e.g. for DOCSIS, modems will respond to, so creating an interface with should work, just adjust that to the IP address range that is actually used by the upstream devices*). Then add the new interface to the WAN zone in the firewall.

*) Make sure they do not use the same range as your openwrt internal network, trying both on is a recipe for unhappiness...

will have to try, but if i understood correctly:
my internal network is in the range.
i need access to 2 sxt , so i should give them IP from, say, range.
the wan interface still remains pppoe and i'd get public IP from the provider.
how to setup the firewall or static routes to be able to access that subnet?

The procedure is documented in the wiki.