Try rebooting after you set the custom rules. It is likely either that or you need to do more work on your phone.
I have been using mine flawlessly since I posted and figured it out in 2017 (hurray for $20 unlimited cellular line)
I started with the cheapo $20 I think gli 350 modem and an old nexus 4for testing. I have since upgrade to the $80 2.4 and 5ghz modem for the 5ghz and for more stable power since it uses a brick and not microusb. I think it is b1300 and a newer phone with more bands (nexus 4 had to be hacked to enable lte and only on band 4).
Something to keep in mind on the phone you must also modify the apn or tethering can still be detected. Additionally carrier branded cellular tethering apps bypass apns and will detect tethering.
I am now using a galaxy s 8, that isn’t tied to any carrier and doesn’t have any carrier firmware. Even that took work modify. (global_tether_dun=0 and the like)
Sorry I can’t be more specific, I didn’t takenotes while working on it and i haven’t touched it (aside from occasional reboots) in ~a year.
If you run linux you can also modify the ttl for your linux box via the command line for testing.