diff --git a/samples/cellular/lwm2m_client/overlay-avsystem-bootstrap.conf b/samples/cellular/lwm2m_client/overlay-avsystem-bootstrap.conf
index 0fef3e148b6a..0e572faa7836 100644
--- a/samples/cellular/lwm2m_client/overlay-avsystem-bootstrap.conf
+++ b/samples/cellular/lwm2m_client/overlay-avsystem-bootstrap.conf
@@ -5,3 +5,7 @@ CONFIG_LWM2M_SECURITY_INSTANCE_COUNT=3
 # Enable TLV, AVSystem uses this as a default.
 # Can be disabled once device dialect is changed from server side.
 CONFIG_LWM2M_RW_OMA_TLV_SUPPORT=y
+
+# AVSystem FOTA defaults to CoAPS so use the same security tag than where our
+# DTLS credentials are stored.
+CONFIG_LWM2M_CLIENT_UTILS_DOWNLOADER_SEC_TAG=35724861
diff --git a/samples/cellular/lwm2m_client/overlay-avsystem.conf b/samples/cellular/lwm2m_client/overlay-avsystem.conf
index b0ccf8c5f21e..6463a75c4624 100644
--- a/samples/cellular/lwm2m_client/overlay-avsystem.conf
+++ b/samples/cellular/lwm2m_client/overlay-avsystem.conf
@@ -3,3 +3,7 @@ CONFIG_LWM2M_CLIENT_UTILS_SERVER="coaps://eu.iot.avsystem.cloud:5684"
 # Enable TLV, AVSystem uses this as a default.
 # Can be disabled once device dialect is changed from server side.
 CONFIG_LWM2M_RW_OMA_TLV_SUPPORT=y
+
+# AVSystem FOTA defaults to CoAPS so use the same security tag than where our
+# DTLS credentials are stored.
+CONFIG_LWM2M_CLIENT_UTILS_DOWNLOADER_SEC_TAG=35724861