Explanation
A router-on-a-stick is a configuration that allows a single router interface to route traffic between multiple VLANs on a network1. A router-on-a-stick requires sub-interfaces to be configured on the router interface, one for each VLAN. Each sub-interface is assigned a VLAN ID and an IP address that belongs to the corresponding VLAN subnet. The router interface is connected to a switch port that is configured as a trunk port, which allows traffic from multiple VLANs to pass through. The router then performs inter-VLAN routing by forwarding packets between the sub-interfaces based on their destination IP addresses.
Inter-VLAN routing is a process that allows devices on different VLANs to communicate with each other.
Inter-VLAN routing can be performed by a router-on-a-stick configuration, as explained above, or by a multilayer switch that has routing capabilities. A multilayer switch does not require sub-interfaces to be configured for inter-VLAN routing; instead, it uses switch virtual interfaces (SVIs) that are associated with each VLAN. An SVI is a logical interface that represents a VLAN on a switch and has an IP address that belongs to the VLAN subnet. The switch then performs inter-VLAN routing by forwarding packets between the SVIs based on their destination IP addresses.
VLAN trunking is a method that allows traffic from multiple VLANs to be carried over a single link between switches or routers. VLAN trunking requires the use of a tagging protocol, such as 802.1Q, that adds a header to each frame that identifies its VLAN ID. VLAN trunking does not require sub-interfaces to be configured on the switches or routers; instead, it uses trunk ports that are configured to allow or deny traffic from specific VLANs. The switches or routers then forward packets between the trunk ports based on their VLAN IDs.
802.1Q is a standard that defines how VLAN tagging and trunking are performed on Ethernet networks.
802.1Q adds a 4-byte header to each frame that contains a 12-bit field for the VLAN ID and a 3-bit field for the priority level. 802.1Q does not require sub-interfaces to be configured on the switches or routers; instead, it uses trunk ports that are configured to support 802.1Q tagging and untagging. The switches or routers then forward packets between the trunk ports based on their VLAN IDs and priority levels.

Tracert is a command-line tool that traces the route of a packet from a source to a destination and displays the number of hops and the round-trip time for each hop. The output shown in the question is an example of a tracert output, which shows five hops with their IP addresses and hostnames (if available) and three latency measurements for each hop in milliseconds. Reference: https://partners.comptia.org/docs/default-source/resources/comptia-network-n10-008-exam-objectives-(2-0), https://www.lumen.com/help/en-us/network/traceroute/understanding-the-traceroute-output.html

A captive portal is a web page that is displayed to a user before they can access the internet or other network resources. This is often used in public or guest networks to present users with a login or terms and conditions page before they can access the internet. If a captive portal is active on the WLAN, it would explain why the IoT sensor is only receiving HTTP redirections when trying to connect to the internet.

The default gateway is the IP address of the router that connects a network to the internet or another network.
The default gateway is usually configured on the devices that need to access the internet or other networks, such as PCs, servers, or routers. If the router was configured with an incorrect default gateway, it would not be able to forward packets to the correct destination, and internet access would not be possible.
The other options are not the most likely causes of the issue. The Ethernet interface is the physical port that connects a device to a network using a cable. If the Ethernet interface was configured with an incorrect IP address, it would cause a problem with the local network connectivity, not the internet access. The loopback address is a special IP address that refers to the device itself, usually used for testing or troubleshooting purposes. If the router was configured with an incorrect loopback address, it would not affect the internet access, as the loopback address is not used for routing packets to other networks. The serial interface is another type of physical port that connects a device to a network using a serial cable, often used for WAN connections. If the serial interface was configured with the incorrect subnet mask, it would cause a problem with the WAN connectivity, not the internet access, as the subnet mask is used to determine the network and host portions of an IP address.
References:
What is a Default Gateway? - HowStuffWorks
What is an Ethernet Interface? - Definition from Techopedia
What is a Loopback Address? - Definition from Techopedia
What is a Serial Interface? - Definition from Techopedia