WARNING: THIS IS UNSUPPORTED. Verizon/Frontier only allows their own equipment on their own network. They can ban you at any time.
The experiment was made on a Lantiq FALC ON sfp stick, which shares the same design with Huawei MA5671A and Nokia G-010S-A. Current price is around $20 on Xianyu (闲鱼, Taobao's C2C version) sans shipping. The stick is quite hard to find in the US -- I had to import them from China.
I bought the stick from https://item.taobao.com/item.htm?id=666376847051 . You may also contact the seller at firstname.lastname@example.org. The seller is the OEM manufacturer, and can provide technical support (which is super important, unless you have access to Lantiq's NDA documentation).
Make sure you are using GPON OLT. If you are still using BPON, call your ISP right now to replace it with GPON. XGS-PON is great, but the ONU stick is very expensive (costs ~$200 each).
Write down the S/N of the ISP's equipment. It should start with 4 characters such as ALCL or FTRO. Change the SN of the stick to the ISP OLT's SN. Do not change anything else. LOID authentication is only used by Chinese ISPs, and PLOAM password is not used.
After that, your stick should go into O5 state in a few seconds. However, you definitely won't be able to DHCP until you set the correct VLAN. Depending on your area, you might need to change VLAN tagging configuration from within the SFP stick.
To identify the VLAN ID, look at ONU_GPE_EXTENDED_VLAN_TABLE first:
Name: ONU_GPE_EXTENDED_VLAN_TABLE ID: 41 no;out tpid;in tpid;vlan rule pointer;dscp pointer 0;0x8100;0x8100; ;1 1;0x8100;0x8100; AA;1
VLAN Rule AA points to ONU_GPE_VLAN_RULE_TABLE.
Name: ONU_GPE_VLAN_RULE_TABLE ID: 42 ;;;enable;;;ethertype filter;;;;;outer;;;;;;;inner;;;;;; no;end;def;two;one;zero;5;4;3;2;1;de enable;de filter;input tpid enable;vid enable;vid filter;priority enable;priority filter;de enable;de filter;input tpid enable;vid enable;vid filter;priority enable;priority filter ... AA; ; ; ;1; ; ; ; ; ; ; ; ;1;1; BB; ; ; ; ; ; ; ; ; ...
BB is your VLAN ID (decimal, to be tagged on GEM port side). Payload should be tagged with dot1Q BB from the GEM port end. Create a sub interface on your router with VLAN BB and see if you can DHCP. If you can get an address, then great, that's all you need.
If you can see some packets from the router, but not able to acquire a DHCP lease, try connecting your original router and release the DHCP lease. Some ISPs would only hand out one active lease per account.
The SFP stick is capable at handshaking at 2.5Gbps (with compatible switches). Unifi devices and a few TP-Link devices were said to be compatible. Alternatively, there are 2.5Gbps SFP to Ethernet media converters available from Taobao.