You don't want to allow discovery on isp-pppoe ... so remove this one:
/interface list member
add interface=isp-pppoe list=discover
Addressing seems OK, so RB should be able to access GPON modem. You can verify by running command
/ping 192.168.1.1 on router itself. If it is, then modem should be accessible already also to LAN clients as your masquerade rule is quite permissive (it masks everything that happens to pass router, I'd restrict it by adding
out-interface=isp-pppoe ... but this really depends on topology of your network). If you do restrict your current masquerade rule, then you have to add another one to make GPON modem reachable from the rest of your LAN:
/ip firewall nat
add action=masquerade chain=srcnat comment="towards GPON modem" out-interface=ether1-WAN
Your firewall needs some reworking ... if your internet access is over isp-pppoe, then firewall filter rules should refer to that port. For example the following rule:
add action=drop chain=forward comment="defconf: drop all from WAN not DSTNATed"\
connection-nat-state=!dstnat connection-state=new in-interface=isp-pppoe
My advice would be to use interface lists ... in case WAN interface gets changed, it's not needed to redo many firewall rules, enough is to update WAN list membership.
You might want to restrict access to GPON modem subnet to a few select LAN clients. Without additional FW rules every LAN (and guest) client will be able to connect to GPON modem.