[Samsung Galaxy A3, A5, A7 (2017) Questions & Answe] Bryan48765:
If it is in your CSC, *#2263#, that is, BAND in non-qwerty/azerty/qwertz/dvorak/landscape orientation keyboards. Select gsm (2g) or wcdma (3g) on stack 2 (sim 2) or band disable. If it is not in your csc, obtain _keystrings.dat from a csc that activates it, rename it to yourcsc_keystrings.dat or sm-a520f_keystrings.dat , place in /system/etc. Try *#2263# again. Yes it is possible in a custom rom, if it was configured. The service mode going in oreo is something else: *#2683662# in the modem firmware, which started in Nougat, and a new feature of Oreo itself that prevented certain software from launching it... it is simply more difficult now... though *#2263# will band lock fine. In a520f it looks like this in 2263.

Select STACK

[1] STACK 1

..Network Setting

..[1] Band Selection

...Band Selection

...[Attention]

...4G/4G is not allowed.

...The opposite is GSM Mode

...Selectable mode:

...Auto/4G/3G/2G

...[1] Automatic[*]

....Automatic Band Selection

....[0] Automatic[*]

....[1] LTE band_1 preferred

....[2] LTE band_2 preferred

....[3] LTE band_3 preferred

....[4] LTE band_4 preferred

....[5] LTE band_5 preferred

....[6] LTE band_7 preferred

....[7] LTE band_8 preferred

....[8] LTE band_12 preferred

....[9] LTE band_13 preferred

....[A] LTE band_17 preferred

.... LTE band_18 preferred

....[C] LTE band_19 preferred

....[D] LTE band_20 preferred

....[E] LTE band_26 preferred

....[F] LTE band_28 preferred

....[G] LTE band_29 preferred

....[H] LTE band_38 preferred

.... LTE band_40 preferred

....[J] LTE band_41 preferred

....------------------------------

....[O] Next Band

.... Automatic Band Selection

...[2] GSM

....GSM Band Selection

....[0] GSM ALL

....[1] GSM 850

....[2] GSM 900

....[3] GSM 1800

....[4] GSM 1900

....[5] GSM 850_900

....[6] GSM 1800_1900

...[3] WCDMA

....WCDMA Band Selection

....[0] WCDMA ALL

....[1] WCDMA_I_2100

....[2] WCDMA_VIII_900

....[3] WCDMA_II_1900

....[4] WCDMA_V_850

....[5] WCDMA_II_1700

...[4] LTE

....LTE Band Selection

....[0] LTE_ALL

....[1] LTE_1

....[2] LTE_2

....[3] LTE_3

....[4] LTE_4

....[5] LTE_5

....[6] LTE_7

....[7] LTE_8

....[8] LTE_12

....[9] LTE_13

....[A] LTE_17

.... LTE_18

....[C] LTE_19

....[D] LTE_20

....[E] LTE_26

....[F] LTE_28

....[G] LTE_29

....[H] LTE_38

.... LTE_40

....[J] LTE_41

....------------------------------

....[O] Next Band

.... LTE Band Selection

...[5] TDS/WCDMA/GSM

....HEDGE Selection

....[1] HEDGE ALL

...[6] LTE/WCDMA/TDS

....LTE/WCDMA/TDS Selection

....[1] LTE/WCDMA/TDS Auto

...[7] LTE/GSM

....LTE_GSM Selection

....[1] LTE_GSM

...[8] TDSCDMA

....TDSCDMA Band Selection

....[0] TDSCDMA ALL

....[1] TDSCDMA_34

....[2] TDSCDMA_39

...[9] TDS/GSM

....TDSCDMA/GSM Selection

....[1] TDSCDMA/GSM Auto

...[A] WCDMA/GSM

....WCDMA/GSM Selection

....[1] WCDMA/GSM Auto

... LTE/TDS/GSM

....LTE/TDS/GSM Selection

....[1] LTE/TDS/GSM Auto

...[C] LTE/WCDMA/GSM

....LTE/WCDMA/GSM Selection

....[1] LTE/WCDMA/GSM Auto

..[3] HSPA Setting

...HSPA Setting

...[1] R4 Only

...[2] HSDPA

...[3] HSUPA

...[4] HSPA_REL7

...[5] HSPA_REL8

...[6] HSPA_REL9 [*]

..[4] Band Disable

...Band Disable Setting

...[0] LTE

....Band Disable Setting

....LTE

....[0] RESET

....[1] LTE_1

....[2] LTE_2

....[3] LTE_3

....[4] LTE_4

....[5] LTE_5

....[6] LTE_7

....[7] LTE_8

....[8] LTE_12

....[9] LTE_13

....[A] LTE_17

.... LTE_18

....[C] LTE_19

....[D] LTE_20

....[E] LTE_26

....[F] LTE_28

....[G] LTE_29

....[H] LTE_38

.... LTE_40

....[J] LTE_41

...[1] WCDMA

....[0] RESET

....[1] WCDMA_I_2100

....[2] WCDMA_VIII_900

....[3] WCDMA_II_1900

....[4] WCDMA_V_850

....[5] WCDMA_II_1700

[2] STACK 2