I live in a fairly modern home (built in early 2000s) and it’s nice but there’s no Ethernet ports. There are landline ports though. Has anybody used a landline to Ethernet adapter before? Or has had a set up like this?

Sure. I used them a lot. The early ones which where just called "modem" and the newer ones which are called "DSL modem" or "ADSL modem". they are great for connecting to an internet access provider when you don't have a fiber connection to your home.

RJ 11 to RJ 45?

Seems like a pretty dumb idea.

Insulation and quality of cables is usually "not good", especially in older houses.

Powerline / PLC / PowerLAN suffers from the same issue - it can work great if the electrician who laid the electric grid made a good job. Otherwise - nope.

I'd recommend giving WLAN a choice - but with proper planning.

E.g. thinking what you want to achieve and where the most likely problems occur (e.g. kitchen - bad isolated microwaves etc.).

If you don't need to span several floors and -/ or live in a house that has "esoteric" walls, should be safe choice.

"Esoteric" walls meaning...
- e.g. filled with staniol / asbest / lead... ( depending on country not as uncommon as one might think ... )
- industry grade walls ( e.g. refurbished factory buildings for living )
- paranoid architects -/ house builders who actually wanted to build a bunker

Do you have conduits for your wires? If so, it would be easy to just add Ethernet cables and sockets.

What kind of "phone" lines do you have? The two wire landline cables or ISDN?

Two wire plain old telephone: You could use DSL. It works over copper from second last century, though not necessarily at high speed. I'm currently unable to find a DSL "uplink" modem (DSLAM) for home usage. Phone cables can be daisy chained, so this can break your setup.
In theory there should be powerline like products for telephone wiring as well.

ISDN: Depending on the configuration, you may have two to four wire pairs, potentially limiting you to 100 MB ethernet. Care must be taken if the ISDN sockets are daisy chained - you should be able to add a switch at each room then. As ISDN and Ethernet sockets are AFAIK patched differently, you will need to check the socket. With all pairs per socket I successfully use gigabit Ethernet over a former ISDN wire.

Just yolo it and drill holes for new cables