A brief application note if you are using the RadioHead library.
To the moment, the RadioHead library offers only a set of four predefined modem configurations. These can be found in RH_RF95.cpp:

    { 0x72,   0x74,    0x00}, // Bw125Cr45Sf128 (the chip default)
    { 0x92,   0x74,    0x00}, // Bw500Cr45Sf128
    { 0x48,   0x94,    0x00}, // Bw31_25Cr48Sf512
    { 0x78,   0xc4,    0x00}, // Bw125Cr48Sf4096

The first of these three bytes is written to configuration register 0x1d, the second to 0x1e and the third into 0x26. Relevant settings include:

RegModemConfig 1 / 0x1D

• signal bandwidth (7.8kHz to 500kHz)
• codingRate 4/4 to 4/8
• implicit Header on/off

RegModemConfig 2 / 0x1E

• spreading factor (64chips/symbol to 4096 chips/symbol)
• CRC on/off

RegModemConfig 3 / 0x26

• 'Low Data Rate Optimization' aka MobileNode
• automatic AGC on/off

Utilizing one of the predefined settings, you would want to  initialize your modem like this:

...
#define ModemConfig RH_RF95::Bw31_25Cr48Sf512
...
rf95.setModemConfig(ModemConfig);
...

And  instead of using one of the predefined settings, you can configure your LoRa mode according your needs:

...
 
RH_RF95::ModemConfig myconfig=  { 0x28,   0x94,    0x04}; // Bw15_6Cr48Sf512 
// will give you
// 15.6kHz bandwidth, 4/8 coding rate, spreading factor  9 (512), low data rate optimization off, AGC on
...
rf95.setModemRegisters(&myconfig);

Now, if this seems too cryptic to you, this can be rewritten into a more human readable form. Fortunatelly, the available options are predefined accordingly in RH_RF95.h:

...
// RH_RF95_REG_1D_MODEM_CONFIG1                       0x1d
#define RH_RF95_BW                                    0xf0

#define RH_RF95_BW_7_8KHZ                             0x00
#define RH_RF95_BW_10_4KHZ                            0x10
#define RH_RF95_BW_15_6KHZ                            0x20
#define RH_RF95_BW_20_8KHZ                            0x30
#define RH_RF95_BW_31_25KHZ                           0x40
#define RH_RF95_BW_41_7KHZ                            0x50
#define RH_RF95_BW_62_5KHZ                            0x60
#define RH_RF95_BW_125KHZ                             0x70
#define RH_RF95_BW_250KHZ                             0x80
#define RH_RF95_BW_500KHZ                             0x90
#define RH_RF95_CODING_RATE                           0x0e
#define RH_RF95_CODING_RATE_4_5                       0x02
#define RH_RF95_CODING_RATE_4_6                       0x04
#define RH_RF95_CODING_RATE_4_7                       0x06
#define RH_RF95_CODING_RATE_4_8                       0x08
#define RH_RF95_IMPLICIT_HEADER_MODE_ON               0x01

// RH_RF95_REG_1E_MODEM_CONFIG2                       0x1e
#define RH_RF95_SPREADING_FACTOR                      0xf0
#define RH_RF95_SPREADING_FACTOR_64CPS                0x60
#define RH_RF95_SPREADING_FACTOR_128CPS               0x70
#define RH_RF95_SPREADING_FACTOR_256CPS               0x80
#define RH_RF95_SPREADING_FACTOR_512CPS               0x90
#define RH_RF95_SPREADING_FACTOR_1024CPS              0xa0
#define RH_RF95_SPREADING_FACTOR_2048CPS              0xb0
#define RH_RF95_SPREADING_FACTOR_4096CPS              0xc0
#define RH_RF95_TX_CONTINUOUS_MOE                     0x08

#define RH_RF95_PAYLOAD_CRC_ON                        0x04
#define RH_RF95_SYM_TIMEOUT_MSB                       0x03
...

So your custom modem configuration can be rewritten into this:

// so instead of 
RH_RF95::ModemConfig myconfig=  { 0x28,   0x94,    0x04};

// you can use
RH_RF95::ModemConfig myconfig =  {  RH_RF95_BW_15_6KHZ | RH_RF95_CODING_RATE_4_8, RH_RF95_SPREADING_FACTOR_512CPS, 0x04};

// where the last parameter (0x04) turns the AGC on

All relevant registers are described also within the SX127x datasheet, available e.g. at

http://www.nicerf.com/Upload/ueditor/files/2017-05-22/SX1276_7_8-402ece57-7162-4950-9b1b-92a1669b7a89.pdf

Happy coding!

dk