问题可能是因为在同一WiFi网络下使用HTTPS时，可能由于证书信任问题导致Android无法连接到Spring Boot后端。为了解决这个问题，可以尝试以下几个步骤：

1.在Spring Boot后端项目中配置自签名证书

@Configuration
public class SslConfiguration {

    private final String KEY_STORE_PASSWORD = "password";
    private final String KEY_STORE = "classpath:yourkeystore.jks";

    @Bean
    public EmbeddedServletContainerFactory servletContainer() {
        TomcatEmbeddedServletContainerFactory tomcat = new TomcatEmbeddedServletContainerFactory() {
            @Override
            protected void postProcessContext(Context context) {
                SecurityConstraint securityConstraint = new SecurityConstraint();
                securityConstraint.setUserConstraint("CONFIDENTIAL");
                SecurityCollection collection = new SecurityCollection();
                collection.addPattern("/*");
                securityConstraint.addCollection(collection);
                context.addConstraint(securityConstraint);
            }
        };

        tomcat.addAdditionalTomcatConnectors(initiateHttpConnector());
        tomcat.addAdditionalTomcatConnectors(initiateHttpsConnector());
        return tomcat;
    }

    private Connector initiateHttpConnector() {
        Connector connector = new Connector(TomcatEmbeddedServletContainerFactory.DEFAULT_PROTOCOL);
        connector.setScheme("http");
        connector.setPort(8080);
        connector.setSecure(false);
        connector.setRedirectPort(8443);
        return connector;
    }

    private Connector initiateHttpsConnector() {
        Connector connector = new Connector("org.apache.coyote.http11.Http11NioProtocol");
        Http11NioProtocol protocol = (Http11NioProtocol) connector.getProtocolHandler();
        try {
            File keystore = ResourceUtils.getFile(KEY_STORE);
            connector.setScheme("https");
            connector.setSecure(true);
            connector.setPort(8443);
            protocol.setSSLEnabled(true);
            protocol.setKeystoreFile(keystore.getAbsolutePath());
            protocol.setKeystorePass(KEY_STORE_PASSWORD);
            protocol.setKeyAlias("tomcat");
            return connector;
        } catch (FileNotFoundException ex) {
            throw new IllegalStateException("Cannot load keystore", ex);
        }
    }
}

其中，KEY_STORE_PASSWORD、KEY_STORE和KEY_ALIAS可以根据实际情况进行修改。此配置类中会同时开启http和https两个连接器。

2.在Android应用中信任后端的自签名证书，并启用网络安全配置

在res目录下新建一个xml文件夹，在其中创建一个network_security_config.xml文件，内容如下：