Issue: I’m testing maximum range with RAK811 shield for Arduino and getting a SF10 maximum uplink messages, but wanted to receive SF11 or SF12 for longer distances. The maximum range is about 200 meters. See the setup below.
Setup: RAK7246 gateway installed in third floor apartment (indoor).
RAK811 node sending 15 byte-payload every 3 seconds.
Frequency band: AS923
Server: TTN V2
Details: I can send you more details if you need
Welcome to RAK forum @aless92 
You should be able to set the spreading factor via AT command - at+set_config=lora:tx_power:1 (SF11) or at+set_config=lora:tx_power:0 (SF12)
Documentation is here:
https://docs.rakwireless.com/Product-Categories/WisDuo/RAK811-Module/AT-Command-Manual/#lorawan-type-at-command
Sending a 15 byte payload every 3 seconds would put you in breach of TTN Fair Use Policy and, more important is illegal. Being marginal on exceeding duty cycle is one thing, this is like going at 1,000km/h outside a school with a 5km/h limit with sharp spikes on the front of the car and a plume of oil smoke coming out the exhaust. See:
https://avbentem.github.io/airtime-calculator/ttn/as923/15
Additionally any node transmitting at SF11 or 12 is required by LoRa Alliance rules to be restricted and commanded to use a lower data rate.
Not only is this absurdly too often, you’re probably asking it to transmit again before it’s done with the receive windows.
At SF 10 your packet takes around half a second and so there’s just time to check classic receive windows 1 and 2 seconds later.
But at SF11 it takes around a second and at SF12 two, so the receive windows won’t finish before you try to make it transmit again fortunately seeming to result in failure.
At slow spreading factors you need to be waiting full minutes between packets, maybe a fair chunk of an hour.
Sometimes people jump on the idea of LoRa before realizing that it’s too slow and limited for their needs.