A loss of APN settings has been recorded in the case of a number of devices including FP3 in the case of an Android version upgrade [1] – you might check other threads with hashtag apn.
When this happens I have found the best thing to do is to ask the carrier to resend the APN settings to the phone. A carrier can change APN for their own reasons; I think / believe carriers assume that APN will always be transmitted to the device dynamically. You can research the APN on your device from
Settings > Network and internet > Mobile network > Advanced > Access Point Names.
Your carrier should be identified correctly with a “radio button” – 
. You can compare the settings on your device with the published APN of your carrier.
Additionally, I find Settings > Network and internet > Mobile network > Advanced > Automatically select network is ON.
I know that your carrier was working previously, but just a thought … as said already, ask your carrier directly to send APN to the device … but you might also check if your carrier features in
[1] Later edit. I don’t think you mentioned, but did you, perhaps, actually have an Android version upgrade ?