Sorry for the late reply, I had to conduct some testings and try to figure out if it will work with Oreo …
I’m pretty confident this will work, and I tried to keep it easy :
- take some screenshots of your current settings, just in case …
- reboot device to TWRP Recovery, then mount System read-write
- from computer, issue a
adb pull /system/etc/apns-conf.xmlto get the conf file - MAKE A BACKUP OF THIS FILE
- still on computer, open the file with a good text editor (I used Notepad++)
- locate the line of your APN (warning : you may have several lines with the same APN name, but different settings; you’ll have to pick the right one …)
- as you wish, either copy the line with a new not-so-fancy APN name, or modify it directly
a. please respect the case ! for example SPN for MNVO is upper-case in Android screen display, but it is lower-case in xml file; take a look a others lines as an example
b. if you need to insert a new parameter, you may want to check others lines for it’s place (order)
c. I found that “protocol” & “roaming_protocol” are useless if using only IPv4 - save the file, respecting the Unix format (EOL LF only)
- put back the conf file to the phone using
adb push apns-conf.xml /system/etc/, unmount System from within TWRP - issue a
adb reboot system - once started, go to your existing APN settings, use 3-dot menu, then “Reset…”
- (you may have to select your APN in the list) check that APN settings are correct
- I’ve found that, sometime, it may help to switch to WiFi, deactivate mobile data, and then deactivate WiFi and activate mobile data
- if you have a “user not authorized …” or similar message, your APN conf file may have a syntax error; please post it here by any way you like
- if you want to revert to your original file, reboot to TWRP and repeat the steps from 9 to 12 with the backup file
As a side note, you may want to read or backup your running APN settings while in TWRP Recovery, unfortunately it has moved several times with the different Android versions 
A good guide is available here : root access - Where are APN configs saved in Android? - Android Enthusiasts Stack Exchange
For information, for my encrypted Pie testing device it was is /data/user_de/0/com.android.providers.telephony/databases/telephony.db (SQLite3), I think that for an Oreo device you can find it in /data/data/com.android.settings/shared_prefs/com.android.settings_preferences.xml
(you will have to mount Data partition if not already).
You can try several settings as long as you with, it’s harmless.
Please remember to hit the “Reset…” in 3-dot menu each time you push a new XML file 
I hope it will help solve your problem !